A hammer is a useful tool. It's used to make tools like axe and pickaxe heads and when with an Anvil you can craft Armor. It also can be used to destroy your own Buildings when equipped. For instance, if there's a Stone Fortress blocking your way out of the mines and you just claimed it, you can delete it with your hammer. To delete the structure, you must equip the hammer and then click the structure. A confirmation will pop up, asking if you'd like to destroy that Building. Click yes to destroy, click no to leave the structure alone. It can be crafted from 1 Handle and 1 Stone.