Here is the full list of all 43 Keep Defending achievements.
Complete the Highguard level on Easy difficulty.
Complete The (Small) Wall level on Easy difficulty.
Complete the Eagle Heights level on Easy difficulty.
Complete the Homestead level on Easy difficulty.
Complete the Homestead level on Hard difficulty.
Play the game during Early Access.
Complete the Eagle Heights level on Hard difficulty.
Complete The (Small) Wall level on Hard difficulty.
Complete the Highguard level on Hard difficulty.
Get the killing shot for 5 dragons.
Complete the Crystal Caves level on Hard difficulty.
Complete the Mutton Valley level on Hard difficulty.
Complete the Crystal Caves level on Easy difficulty.
Complete the Mutton Valley level on Easy difficulty.
Complete the Homestead level on Normal difficulty.
Complete the Eagle Heights level on Normal difficulty.
Complete The (Small) Wall level on Normal difficulty.
Complete the Highguard level on Normal difficulty.
Complete the Crystal Caves level on Normal difficulty.
Complete the Mutton Valley level on Normal difficulty.
Complete the Homestead level on Insane difficulty.
Complete the Eagle Heights level on Insane difficulty.
Complete The (Small) Wall level on Insane difficulty.
Complete the Highguard level on Insane difficulty.
Complete the Crystal Caves level on Insane difficulty.
Complete the Mutton Valley level on Insane difficulty.
Complete the Salt Town Docks level on Easy difficulty.
Complete the Salt Town Docks level on Normal difficulty.
Complete the Salt Town Docks level on Hard difficulty.
Complete the Salt Town Docks level on Insane difficulty.
Complete the Sand Trap level on Easy difficulty.
Complete the Sand Trap level on Normal difficulty.
Complete the Sand Trap level on Hard difficulty.
Complete the Sand Trap level on Insane difficulty.
Complete the Tower Of Stars level on Easy difficulty.
Complete the Tower Of Stars level on Normal difficulty.
Complete the Tower Of Stars level on Hard difficulty.
Complete the Tower Of Stars level on Insane difficulty.
Complete the King's Ending level on Easy difficulty.
Complete the King's Ending level on Normal difficulty.
Complete the King's Ending level on Hard difficulty.
Complete the King's Ending level on Insane difficulty.
Complete all of the levels successfully in the main campaign.