The hammer is one of the most useful tools in the game. Hammers are used to mine gold from rocks, attack enemies and build stuff (Castle Mode) They must be researched but there are no requirements to unlock them.
Hammer is made with 1 - 5 wood, depending on how big your tribe is. To research it, get enough exp until the bottom research bar pops up. Then click the hammer and it will move to the left side of the screen, where it will stay until you die. To give your minion a hammer, select the icon on the left side of the screen, and then click and available minion. Hammer is a melee tool and so it can be paired with shields.
Minions with hammers, known as miners, will automatically move to mine rocks when they are close enough. They will hammer the rock until it is fully mined and drops gold, and then will then collect the gold and move onto the next rock that is within their range.
In battles, miner minions will move to any available minion and engage them in a 1v1, just like with all other melee tools. Like always, knight minions are prioritized and hunter minions are attacked last.
Also, in Castle mode they can build structures in exchange of 7 wood, its really useful in mid-game, since it's needed to build castles.
Tips and Tricks
Hammer is used to get gold, one of the most crucial resources in the game. Getting miners is very helpful, as you can trade gold for all other resources at the special buildings. Also, combat items like sword, shields, and helmets are and can only be bought with gold. Because of this it is very recommended to research hammer.