If any single enemy troop manages to reach a wall segment, they will begin to tear it down. Stronger (that is, thicker or larger) segments last longer under the assault, but eventually they crumble down and have to be constructed again. Special defences on walls (i.e. cauldrons of boiling oil) automatically activate, killing an enemy troop that reaches such a wall segment, but only if they have been placed into the wall in the design phase. Also, if an enemy soldier reaches a moat section, they have to ford it first in order to pass through.