A guard tower is like a really tall building with little windows in it. People can stand in the tower and see very far away. The tower is usually built near a place that needs to be kept safe, like a jail or a military base. The guards in the tower can watch out for bad guys and make sure everything is okay. It's like they're playing a game of "I Spy" but with really important things. If they see something bad happening, they can tell people who can stop it. The guard tower is like a lookout spot where people keep watch to make sure everyone is safe.