The Borgia Towers are back from Brotherhood, but this is another
element of gameplay that has been tweaked and expanded; Constantinople lets you take
control of Assassin Dens around the land (raid the den, extinguish the Templars in the vicinity and have a nice place to call your own), but once the Dens are
acquired it's up to you and your band
of assassins to hold the fort, as the Templars won't be best pleased that you've stolen their digs.