towers and catapults

The peasant can build scout towers. These towers basically do nothing, but they can quickly evolve on their own (without further help of the peasant) to a more useful tower: either the guard tower (requires a sawmill) or the cannon tower (requires a blacksmith). In the early game, the scout tower is sometimes built "just in case" of a rushed attack, with a smaller investment than the cost of a fully equipped tower.

The guard tower shoots arrows, like an archer, while the cannon tower only attacks ground units. Both don't require any food nor empty space (meadow).

A counter against towers is the catapult. The catapult is very fragile and cannot even defend itself against a unit in the same square. It moves very slowly too. On the other hand, it can strike units and buildings in an adjacent square, so towers are not a problem for a catapult.

Comment: at the moment, there are only 2 kinds of "interactive" distance: in the same square (sight and attack) or in an adjacent square (no sight, no attack, except for catapults). There could be an intermediate distance where units can see an intruder but doesn't attack. This might complicate the game too much, but it might be interesting.