This next one is a pretty small, simple, average 2D game but one that's worth giving a shot. If you're really low on funds and want a game that's completely free and has zero ads, this is it.
Sacred Shield has polished 2D graphics where you'll play a Knight trying to save his friends. Use the Sacred Shield to block incoming attacks and keep everyone safe. This game has a leaderboard, simple touch controls, and again, no ads anywhere.