A facade with windows is to be modeled for an outdoor view. The windows are half-rounded. 

The frames on the outside and the window sill are to be taken into account too. 

The following image shows one possible way of doing this. Use is made of nested Boolean groups in order to implement the final result. 

On the inside, this is a union (wall with frames and their window sills). Holes for the window openings are subtracted from these.