Here is the full list of all 18 Planet Stronghold achievements.
Complete the male romance subplot with Rebecca Fox
Complete the male romance subplot with Rumi Kai
Complete the male romance subplot with Prince Cliff
Complete the female romance subplot with Tom Shatz
Complete the female romance subplot with Damien Vargas
Complete the female romance subplot with Michelle Lafleur
Complete the game playing on the King's side
Complete the game playing on Cliff and Shiler's side
Defeat a enemy boss at maximum difficulty level
Complete at least one romance subplot
Maximize at least one skill (value >= 199)
Kill 100 alien enemies in the game
Kill Marada's Queen Shiler
Win all the fights in the rebels' arena
Never lose a single fight in the whole game
Complete the optional Philipp side quest
Have Damien fight on your side in the final battle
Complete at least one homosexual romance subplot