Looks interesting. I was thinking each level could have several win conditions or "missions/quests" so to speak? That would give some variety. Ideas for different mission types could be.
*kill all enemies.
*protect resource - enemies will keep moving towards one point that you have to protect.
*delivery - bring something from point A to B
*sneaky - go from point A to B undetected.
*find something. Scout the map. Hidden item is not revealed until you are 2 tiles away.
Maybe something like that would be fun gameplay?