A wall gun is a very old-fashioned kind of gun that was specifically designed to be mounted, or hung up, on a wall in a house or a fortress. Imagine a really big and long gun that is too heavy for someone to hold up and shoot for a long time.
It was like a decoration, but also really useful for protection during war times because it could shoot a really long distance! Think about shooting a ball with a slingshot, but instead, it shoots a really heavy metal ball that can hurt people and destroy things far away.
When someone wanted to use the wall gun, they would take it down from the wall and set it up to shoot. People needed to be really careful with wall guns because they were pretty dangerous and could hurt someone if they were not careful.
Nowadays, wall guns are not used anymore because we have newer and better weapons that are easier to use and safer for everyone.