You will occasionally get an option to fight a monster and put it into a jail. The monster will award you with resources over time, depending on its level. The jail starts with only three monster cells. You must save 1.2 million coins to expand it further. However you can sell the monsters that have been captured to make room for new monsters. For example, the 1 star monster is worth 100,000 coins when sold. Consider selling monsters to keep one cage available when a new monster appears. Although you might get less low level materials, you will earn money faster.