Illustrator :: How To Connect Multiple Endpoints Into One Straight Line

I made a silhouette of buildings and I"m wanting to connect the bottom open endpoints into one straight line. The reason I'm doing this is because I want to add a fill to the silhouette as a whole instead of doing them one by one.
 
I tried joining paths but it can only be two open endpoints at a time. I have many open endpoints.
 
The pen tool only joins two open endpoints. If I try to continue to try to add another endpoint to the same line, the pen tool becomes a "minus anchor point" tool and messes up the shape of the buildings.
 
Of course, I thought of just making each building separate and grouping them together. However, if I do that, a issue occurs because I have certain buildings that are "in front" of each other (not in the sense of layers, but the image as a whole). I purposely made the sihlouette look like some buildlings are in front of each other and some are behind each other.