Cut circular shape out of 3d map mesh


I would’t even call myself a beginner but I have been experimenting a little bit and wanted to try different things I couldn’t find any tutorials about.
I’m trying to cut this shape out of this mesh of buildings just like I did with the terrain mesh.

But when I try to do the same with the Buildings mesh it doesn’t work since it’s not a closed mesh right? Here is the result of mesh splitting it:

Can anyone tell me how I should proceed?

I’m trying to join the two meshes and insetting it into a solid.

Thanks in advance!