Here is the full list of all 12 Medieval Defenders achievements.
Didn't let a single enemy reach the outpost throughout a level.
Completed a level without improving towers.
Place all towers available in the episode
Placed all types of towers accessible in the episode on a single tile.
Used all the different types of bonuses on a level.
Sold 10 towers on a single level.
Installed all types of towers accessible on a level.
Unlocked a tile for elixir on any level.
Used bonuses 10 times on a single level.
Completed a level with 10 heart remaining.
Collected 10,000 coins in a single level.
Destroyed 20,000 enemies throughout the entire game.