Castle Invasion: Throne Out

There's a level not quite sure which where it's nothing but dwarfs. But this should unlock by level 20

Dwarves first appear on level 3 - On my count approx 13 Dwarves made an appearance. I unlocked this achievement simply by replaying the level 3 additional times after my first initial run. (4 runs total) This is quickest and earliest opportunity to do this if you're just starting the game.