Multiplayer rule: x towers allowed per player
Now of course you can shoot towers with ranged units safely etc, but its way too hard if there is a swarm of bowmen and crossbowmen guarding the towers. Wich results in extremely passive game.
To avoid that Id suggest using this rule: players cant have more than 4 towers at any moment in the game.
You can make up to 4 towers, if you lose one you can make a new one. If you have more than 4 towers at any moment in the game you instantly lose the match. Towers being built also count as towers. So you cant even start building a new tower if you already have 4. This should result in much less passive games.
You might still want to have some towers though, otherwise sending 1 militia into town is too effective. If you have 2-4 towers covering the key points in town then its not too easy to harass.