As most of you know, enemies pretty much can't do anything when you build a wall/fence around your town. This is especially the case if there isn't even a door for ogres to break open. Here I would like to suggest some ways for enemies to finaly be a threat to you, even if you are fully surrounded by walls.
Different Enemy Types:
- Flying Units: The most obvious way to make your town at least a little bit vulnerable to enemy attacks is by implementing flying enemies. Maybe some very aggressive bird-style monsters, or even a completely new race (the birdmen). They can fly over obstacles and therefore reach any place in the open, even if there are walls
- Climbing Units: To make goblins/orcs a threat, they could start to learn climbing walls. Maybe at a very slow pace, so that it gives you some time to organize before the fight starts.
- Wall Destroying Units; The last option I present is, that certain enemy units can destroy walls. They dont destroy it completely, they only put a hole into the wall (walls and building alike), which can be easily repaired by mason/carpenter.