Two monsters can be placed in the building. Monsters placed here will stop wandering around the player's island. Whenever the player clicks on the building, or if it is already selected and the player clicks the button "Action," then the two monsters will perform their animations. Also, the crystal at the top of the tower shines like a lighthouse beacon.
The building is 4x4. If it is moved, the two monsters go with it. Placing monsters in the surveillance tower (or any other building) will prevent those monsters from being moved into Storage, or sacrificed for power-up material. They can still be used for arena, guild wars, dungeons, etc.