1: You may have put a wall THROUGH another wall, causing this odd effect. My advice: NEVER overlap brushes
2: Never EVER make a large brush and hollow it. Ever. Not for any map. While it prevents leaks, the map will probably take hours to compile. It's best to make it in a similar way to an indoor map, but with outdoor features. Use sky brushes in exactly the same way as a ceiling.
Hope this helps, and remember, don't post your first map