Your castle wall protects your Castle and can be upgraded to level 5 with Resources. Every time you upgrade them, it increases the protection by an additional 20% each time. You can equip defensive tools on it, with the number depending on the level of the wall. They are always required to be upgraded before you can upgrade Towers or your Castle gate. It is one of the few structures you do not need to build and it can be upgraded instantly with no building time. Every time you get an expansion, the wall will automatically wrap around it.