Mittelborg: City of Mages (UP)

Quite simple you only have to upgrade the Garrison to Level 3 for the Buildings Menu so that it can house three mages.

Be aware that the whole game is buggy and seems to treat your first 2 Lifes like a Tutorial without the ability to unlock achievements.
If you like me fulfill this condition during the totorial you need to delete your save, redo the Tutorial and do the condition again.