I was wondering if you need to put villagers in a castle after you capture it mount and blade PC game

A captured castle has no garrison, meaning if you leave it alone, anyone can capture it by just beseiging it long enough to make some ladders or seige tower and get inside. If you want the place to be protected while you leave, you need to garrison it with troops from your own personal army. However if the castle is awarded to another lord, these troops will be lost to you.

You can garrison it with peasants if you for some reason have them in your retinue (recruited them after taking them prisoner or after rescuing them from captivity) but they of course won't be able to protect it in the same way as actual soldiers.