How come this knowledge went completely forgotten around 1400 AD, where the cities turned to slums and the only places that showed some evolution were the castles ?

Have you been getting your information from Game of Thrones? The 15th century saw an acceleration in the development of humanities, arts, and sciences, and this included urban planning and architecture. Observe, for example, the 15th century paintings Città Ideale, or "Ideal City" composed by artists like Piero della Francesca, Fra Carnevale, and unnamed central Italian painters as an exercise in the expression of new values of proportion, perspective, and urban planning. In the next two centuries, city planners (especially in Italy, but in other parts of Europe too) devised new urban spaces which ranged from planned towns to massive hydrographic works. In Venice, for example, in the early 17th century city authorities undertook a massive project of waterway redirection in order to render the city of Venice more sanitary.

Th the 15th century in particular, the construction of the first structures dedicated to healthcare took place while there was also an explosion of urban construction and expansion in cities like Venice, itself an astounding feat of post-Roman human engineering.

Many people have a preconceived notion about post-Roman Europe; and how after the fifth century there was a decline in "knowledge," whatever that means. However, the very concept of "Renaissance" in the fifteenth and sixteenth centuries which purported to be a re-birth of the arts and sciences was the invention of a specific group of people. In reality, even though the smaller and more fragmented post-Roman political entities of Europe would not be able to match the massive scale of Roman engineering and city planning, construction of buildings like cathedrals did keep moving forward in Europe.