Raider have 3 horse archer units. I think the only way for defenders to win is to use siege mantlet 2 step + pikemen 3 step to kill archer units first. The defenders don't have chance to win if rounds are not limited.
I suggest to add a trigger to balance the map:
If raiders can't destroy all defenders within
X turns, defenders win the game immediately.

X is around 1x~30 turns?