Although Krier is well known for his advocacy of classical architecture and the reconstruction of traditional “European city” models, a close examination of his work shows, in fact, a substantial shift from an early modern rationalist approach such as his project for Bielefeld University of 1968, to a more vernacular and classical approach, both formally and technologically. [10]​ The project that marked a major turning point in his campaign attitude towards the reconstruction of the traditional European city was his unexecuted plan for the "reconstruction" of his 1978 hometown of Luxembourg, in response to the city's modern urban renewal. He later masterfully planned the new Cité Judiciaire of Luxembourg, which was to be architecturally designed by his brother Rob Krier, work that he carried out between 1990 and 2008.[11]​.

In 1990, of the nine invited experts, he was the only one to support the Dresden citizen initiative to rebuild the historic Dresden Frauenkirche "Frauenkirche (Dresden)") and the Historische Neumarkt area "Neumarkt (Dresden)") and, in 2007 he supported the Frankfurt Altstadt Forum,[12] also a citizen initiative that arose for the reconstruction of the historic "Hühnermarkt", all This against strong opposition from German politicians and architects, who did not accompany the citizens of that city of Frankfurt.

Krier has applied his theories in detailed large-scale plans for numerous cities in the Western world. These include the unrealized plans for Kingston upon Hull (1977), Rome (1977), Luxembourg "Luxembourg (city)") (1978) which was his most comprehensive masterplan focused on mitigating urban sprawl and repairing the city centre, West Berlin (1977–1983), Bremen (1978–1980), Stockholm (1981), Poing Nord, Munich (1983), a masterplan completed in 2000 for Washington D.C. (1984) commissioned by the Museum of Modern Art in New York, Atlantis, a neoclassical neighborhood for intellectuals and artists in Tenerife (1987),[13] Area Fiat, Novoli (Florence), Italy (1993), Corbeanca, Romania (2007), and High Malton Masterplan for Fitzwilliam Estate, Yorkshire, England (2014).

Krier has designed plans commissioned by public administrations, including the redevelopment of Tor Bella Monaca, a run-down suburb of Rome (2010), and a long-term redevelopment policy plan for the municipal area of Cattolica, Rimini, Italy "Cattolica (Italy)") (2017),[14][15] he was able to apply similar principles to build developments such as Knokke, Heulebrug, Belgium (1998), completed without his address; and his masterplan for the Newquay growth area (2002-2006), Cornwall, UK, continued after his resignation, run by the firm Adam Associates.

As of September 2022, Krier was designing plans for Poundbury, Dorset, United Kingdom (1988-present), for Ciudad Cayalá "Ciudad Cayalá (Guatemala)"), Guatemala City (2003-present); El Socorro and Nogales, two new urban neighborhoods for Guatemala City (2018-present), and the redevelopment of the closed Fawley Waterside power station, Southampton, United Kingdom (2017-present), which obtained outline planning permission in July 2020, with construction beginning in 2022 with the first homes expected to be available in 2024, as well as the masterplan for a new town, Herencia de Allende, nearby from San Miguel de Allende, Mexico (2018-present).

Krier agreed with the view of the late Heinrich Tessenow that there is a close relationship between the economic and cultural wealth of a city, on the one hand, and the limitation of its population, on the other. But this is not a mere hypothesis, he maintains, but rather a historical fact. The measurements and geometric organization of a city and its neighborhoods are not the result of a mere chance or accident or simply of economic necessity, but rather represent a civilizing order that is not only aesthetic and technical, but also legislative and ethical.

Krier states that "the whole of Paris is a pre-industrial city that still functions, because it is very adaptable, something that the creations of the 20th century will never be. A city like Milton Keynes cannot survive an economic crisis, or any other type of crisis, because it is planned as a mathematically determined social and economic project. If that model collapses, the city will collapse with it." Thus, Krier argues not only against the contemporary modernist city by de facto arguing that places like Los Angeles are not cities, but also against the tendency towards gigantism in urban growth, evident in the explosive scale of urban networks and buildings in European cities throughout the century, as a result of the concentration of economic, political and cultural power.[16] In response to this, Krier proposes the reconstruction of the European city, based on polycentric settlement models that are not dictated by the scale of cities. machines, but by the human scale both horizontally and vertically, of self-sufficient mixed-use neighborhoods that do not exceed 33 hectares or 81.5 acres, neighborhoods that can be crossed in 10 minutes on foot, with building heights of 3 to 5 stories or 100 steps, which can be climbed comfortably on foot, and that are limited not by mere administrative borders, but by vehicular boulevards, walkable roads and parks, either by pedestrian or bicycle paths. Cities, then, will grow through the multiplication of independent urban neighborhoods, not through horizontal or vertical overextensions of already established urban centers.

Krier brings together his critiques and points out concepts in the form of series of annotated drawings and didactic diagrams, often written in his own handwriting, eventually compiled in his book Drawings for Architecture, such as the concept of the urban in his 1983 diagram of a truly urban city: RES PUBLICA + RES PRIVATA. There he conceives the basic urban fabric, made up of buildings and private uses, as an object of popular local design, and exceptional public and institutional buildings, as objects of classical architecture and located in privileged places, in squares and in the focus of everyone's vision.

The principle behind Krier's writings has been to explain the rational foundations of architecture and the city, stating that “In the language of symbols there can be no misunderstanding.” That is, for Krier, buildings have an order and a rational type: a house, a palace, a temple, a bell tower, a church, but also a roof, a column, a window, etc., what he calls “nameable objects.” As projects grow, he goes on to argue, buildings should not grow, but rather divide, so for example, in his unrealized project for a school in Saint-Quentin-en-Yvelines (1978), France, the school became a “miniature city.”

Krier proposes very varied functional programs within each block and plot. For him, building design must always be typologically or tectonically justified, and the variety of building types and volumes must reflect this functional variety in an obvious and natural way. In summary, any uniformity or gratuitous variety must be avoided, designing lots of neighboring buildings with dimensional, functional and, therefore, formal variety, in such a way that they generate networks of public spaces composed of public streets, squares, avenues, boulevards, parks. For Krier, it is essential to simultaneously compose harmony between urban blocks and the inseparable public spaces that are generated between them.

In the search for such typological architecture, Krier's work has been called “an architecture without style.” However, it has also been noted that the appearance of his architecture is very similar to Roman architecture, which he then places in all his projects, whether in central London, Stockholm, Tenerife or Florida.[17] He has defended the work of architect Albert Speer, distinguishing him from the regime for which he worked.[18]​.

Krier has published manifesto texts on specific topics, many of which are available online:[19]​.

Although Krier is well known for his advocacy of classical architecture and the reconstruction of traditional “European city” models, a close examination of his work shows, in fact, a substantial shift from an early modern rationalist approach such as his project for Bielefeld University of 1968, to a more vernacular and classical approach, both formally and technologically. [10]​ The project that marked a major turning point in his campaign attitude towards the reconstruction of the traditional European city was his unexecuted plan for the "reconstruction" of his 1978 hometown of Luxembourg, in response to the city's modern urban renewal. He later masterfully planned the new Cité Judiciaire of Luxembourg, which was to be architecturally designed by his brother Rob Krier, work that he carried out between 1990 and 2008.[11]​.

In 1990, of the nine invited experts, he was the only one to support the Dresden citizen initiative to rebuild the historic Dresden Frauenkirche "Frauenkirche (Dresden)") and the Historische Neumarkt area "Neumarkt (Dresden)") and, in 2007 he supported the Frankfurt Altstadt Forum,[12] also a citizen initiative that arose for the reconstruction of the historic "Hühnermarkt", all This against strong opposition from German politicians and architects, who did not accompany the citizens of that city of Frankfurt.

Krier has applied his theories in detailed large-scale plans for numerous cities in the Western world. These include the unrealized plans for Kingston upon Hull (1977), Rome (1977), Luxembourg "Luxembourg (city)") (1978) which was his most comprehensive masterplan focused on mitigating urban sprawl and repairing the city centre, West Berlin (1977–1983), Bremen (1978–1980), Stockholm (1981), Poing Nord, Munich (1983), a masterplan completed in 2000 for Washington D.C. (1984) commissioned by the Museum of Modern Art in New York, Atlantis, a neoclassical neighborhood for intellectuals and artists in Tenerife (1987),[13] Area Fiat, Novoli (Florence), Italy (1993), Corbeanca, Romania (2007), and High Malton Masterplan for Fitzwilliam Estate, Yorkshire, England (2014).

Krier has designed plans commissioned by public administrations, including the redevelopment of Tor Bella Monaca, a run-down suburb of Rome (2010), and a long-term redevelopment policy plan for the municipal area of Cattolica, Rimini, Italy "Cattolica (Italy)") (2017),[14][15] he was able to apply similar principles to build developments such as Knokke, Heulebrug, Belgium (1998), completed without his address; and his masterplan for the Newquay growth area (2002-2006), Cornwall, UK, continued after his resignation, run by the firm Adam Associates.

As of September 2022, Krier was designing plans for Poundbury, Dorset, United Kingdom (1988-present), for Ciudad Cayalá "Ciudad Cayalá (Guatemala)"), Guatemala City (2003-present); El Socorro and Nogales, two new urban neighborhoods for Guatemala City (2018-present), and the redevelopment of the closed Fawley Waterside power station, Southampton, United Kingdom (2017-present), which obtained outline planning permission in July 2020, with construction beginning in 2022 with the first homes expected to be available in 2024, as well as the masterplan for a new town, Herencia de Allende, nearby from San Miguel de Allende, Mexico (2018-present).

Krier agreed with the view of the late Heinrich Tessenow that there is a close relationship between the economic and cultural wealth of a city, on the one hand, and the limitation of its population, on the other. But this is not a mere hypothesis, he maintains, but rather a historical fact. The measurements and geometric organization of a city and its neighborhoods are not the result of a mere chance or accident or simply of economic necessity, but rather represent a civilizing order that is not only aesthetic and technical, but also legislative and ethical.

Krier states that "the whole of Paris is a pre-industrial city that still functions, because it is very adaptable, something that the creations of the 20th century will never be. A city like Milton Keynes cannot survive an economic crisis, or any other type of crisis, because it is planned as a mathematically determined social and economic project. If that model collapses, the city will collapse with it." Thus, Krier argues not only against the contemporary modernist city by de facto arguing that places like Los Angeles are not cities, but also against the tendency towards gigantism in urban growth, evident in the explosive scale of urban networks and buildings in European cities throughout the century, as a result of the concentration of economic, political and cultural power.[16] In response to this, Krier proposes the reconstruction of the European city, based on polycentric settlement models that are not dictated by the scale of cities. machines, but by the human scale both horizontally and vertically, of self-sufficient mixed-use neighborhoods that do not exceed 33 hectares or 81.5 acres, neighborhoods that can be crossed in 10 minutes on foot, with building heights of 3 to 5 stories or 100 steps, which can be climbed comfortably on foot, and that are limited not by mere administrative borders, but by vehicular boulevards, walkable roads and parks, either by pedestrian or bicycle paths. Cities, then, will grow through the multiplication of independent urban neighborhoods, not through horizontal or vertical overextensions of already established urban centers.

Krier brings together his critiques and points out concepts in the form of series of annotated drawings and didactic diagrams, often written in his own handwriting, eventually compiled in his book Drawings for Architecture, such as the concept of the urban in his 1983 diagram of a truly urban city: RES PUBLICA + RES PRIVATA. There he conceives the basic urban fabric, made up of buildings and private uses, as an object of popular local design, and exceptional public and institutional buildings, as objects of classical architecture and located in privileged places, in squares and in the focus of everyone's vision.

The principle behind Krier's writings has been to explain the rational foundations of architecture and the city, stating that “In the language of symbols there can be no misunderstanding.” That is, for Krier, buildings have an order and a rational type: a house, a palace, a temple, a bell tower, a church, but also a roof, a column, a window, etc., what he calls “nameable objects.” As projects grow, he goes on to argue, buildings should not grow, but rather divide, so for example, in his unrealized project for a school in Saint-Quentin-en-Yvelines (1978), France, the school became a “miniature city.”

Krier proposes very varied functional programs within each block and plot. For him, building design must always be typologically or tectonically justified, and the variety of building types and volumes must reflect this functional variety in an obvious and natural way. In summary, any uniformity or gratuitous variety must be avoided, designing lots of neighboring buildings with dimensional, functional and, therefore, formal variety, in such a way that they generate networks of public spaces composed of public streets, squares, avenues, boulevards, parks. For Krier, it is essential to simultaneously compose harmony between urban blocks and the inseparable public spaces that are generated between them.

In the search for such typological architecture, Krier's work has been called “an architecture without style.” However, it has also been noted that the appearance of his architecture is very similar to Roman architecture, which he then places in all his projects, whether in central London, Stockholm, Tenerife or Florida.[17] He has defended the work of architect Albert Speer, distinguishing him from the regime for which he worked.[18]​.

Krier has published manifesto texts on specific topics, many of which are available online:[19]​.