Gatehouses
Gatehouses are the main routes in and out of your castle. It is always a good idea to have several men stationed on top of one, in case your opponent makes a surprise assault to capture them with assassins. The gatehouse will only let your own and allied units through it, but if captured only enemies will be allowed through.
Tip: It is always a good idea to have several men stationed on top of a gatehouse, in case your opponent makes a surprise assault to capture them with ninjas.