Fortresses are larger than Castles, but not quite as large as Citadels. Counter Raids sent by Fortresses can consist of up to 10 or more more units. Clicking on a Fortress returns the message, "This is a higher level military structure, beware."
Occupants: 25-50 Elves, levels 100-600 (chance of encountering a level 75-300 Paladin).
Commonly referred to as an "F3" (Fortress Tier 3) Settlement.