What Koveras forgot to explicitly point out is that not killing monsters is actually superior for building. The 1 remaining HP allows a player to swing once and get the 'attacked' and 'killed' portion of the HP advancement roll, even if it's a mule that cast the wall spell. This is the standard way veterans game the building system.

I've no good idea for solving the monsters & walls issue at this time. I still believe it's Bait that's the problem. The abuse to group up monsters that don't fight back will always lend itself to abuse.