The tower defense genre is very popular and works especially well on Android smartphones.
33 RD: Random Defense
This exciting game requires the player to create a team of animals and wizards to protect themselves against lurking enemies. If the monsters reach 33 in number, then you have no chance against them. To prevent them from getting to that number, you’re required to summon your best forces of animals and wizards. You earn a reward after completing 100 stages.