[3] During his time at the Academy, Lilljekvist worked with the art historian Gustaf Upmark in documenting Gripsholm Castle.
An opposing ideology that would eventually come to dominance in Sweden in the early 20th century was to restore buildings in a way that showed respect for its entire history.
Lilljekvist's work was severely criticized in an article in the newspaper Dagens Nyheter by the writer Verner von Heidenstam, who suggested that a sign should be put at the Castle saying in mock-archaic Swedish, that "An̄o Domini 1893 gjordes detta gambla hus ændnu gamblare" ("An̄o Domini 1893 this old House was made even older").
In the end, he produced a design that took some of the volumes from Neo-Baroque architecture but with ornamental and sculptural forms borrowed from Swedish nature but influenced by continental Art Nouveau, which, by the time the building was completed in 1908, had already started to go out of fashion.
In Westman's view the decoration of the building should be like the ornaments of a knight's armour rather than like the bands and flowers of his lady's dress.
In the following years, he worked on several projects for Swedish embassy buildings abroad, and the last decade-and-a-half of his life was entirely dedicated to urban planning.