There are only two great fortresses that were constructed by and home to the enderborn. Enderbor and Enderbrine's Lair.
Enderbor, the mightiest kingdom in all of the endarian race. Constructed within a hollowed out mountain outside the walls of the oldest city and capital of the enderborian hold, Doug. The city was named after what the farlanders used to believe the enderdragon's was named: Douglas. Enderbor is the home of Alduin and will be the home to his descendants.
Enderbrine's Lair can be found in the end after the enderdragon and it's reign has been destroyed. Hints the name of the fortress, it is the cozy dark home the mystical enderborn, Enderbrine. The fortress has a lava moat, surrounding the tall dark obsidian walls that compound the keep and the rest of the fortress' glory.