Subsequent stages of the building's expansion are connected with the development of Warsaw – the castle took on its Gothic forms in the first decades of the 15th century when Prince Janusz I the Elder moved the capital of the Duchy of Mazovia from Czersk to Warsaw. It was thanks to him that between 1407 and 1410, a three-storey brick palace, the so-called Great House (Curia Maior) was built here, housing not only the princely suites but also the hall used for many years as the venue of the Mazovian Sejm sessions.
However, the greatest transformation of the Warsaw princely residence took place in the second half of the 16th century, thanks to Sigismund Augustus, who chose the Warsaw building as his new residence. He entrusted its expansion to renowned architects of the time, the Silesian Jakub Parr (who had just before turned the castle in Brzeg into a pearl of Renaissance art) and the Italian Giovanni Baptista Quadro, the creator of the Poznań town hall. When the Renaissance architects began work here, the Warsaw castle consisted of the Great House with the Grodzka Tower, the southern building and the Minor Court.
As part of the reconstructions and extensions started in the second half of the 16th century, amongst others, the New House was built, which housed the king's private flats, the Great House was transformed into a parliament building, the Lesser House was rebuilt for the needs of Anna Jagiellon (amongst others, a large dining room and a chapel were created here) and the whole establishment was given a more coherent expression. Work on the transformation of the castle from a mediaeval defensive structure into an ornamented royal residence continued in subsequent years when successive Italian architects were engaged in rebuilding the castle interiors.