Guard tower is a town building which you can climb to access to the top, where a small lever lets you use a ballista to bring down a flying goblin. That rewards you with 4 coins.
-You can delete one of the four doors of the tower and add a reward inside as a secret. The one facing the opposite of the player is the most adequate.
-The ballista stand on top contains logic for turrets, make sure to give it a peak or even select the Ballista and save it as an assembly.