Dig ditches around your outer walls to form a perimeter, generally 3-5 feet wide. Trolls cannot jump, (nor can any other mob in this game). The only way a monster can get past a ditch is if they can fly. Once something wanders down into your pit, you can either 1. Kill it with the bow if you want or 2. Ensure you have an "out" from the pit and once the siege ends, the mobs will simply wander out and away from your home base.

Monsters who get into the pit by the way will keep making a beeline towards you. Lure them all to one particular side of your pits and then run to the opposite side, making sure to keep distance. The troll or archers or whatever will never even damage your walls or swing their weapons if they aren't in range of the player.