Hello Archon,

You click the edit button at the right corner of your game screen

http://prntscr.com/deeudm

After you click edit you click on the wall you want to get rid of and click destroy if you want it gone completely or move if you wish to move it elsewhere! Then you click save and you can build the tower you want!

http://prntscr.com/deesjd

Hope I have helped you!