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.