1) Someone who is so antagonistic, negative, closed-off, and just overall cunty that they are physically and mentally exhausting to be around. You can identify a cuntfortress when you would rather chop your own dick off than listen to one more whiny comment about how "everyone here is fucking lame" or watch them pout and generally bring the entire vibe down while they adamantly refuse to enjoy themselves.
Often seen whining at their friend/significant other about how much they want to go home because they aren't having fun when in fact, they are Fun Black Holes, sucking in all potential fun and excitement out of the room and into their vortex of insecurity and despair. Even attempts to include the cuntfortress in on the fun will result in excessive eye-rolling and insults at your expense. Often these hateful comments and negative facial gestures are all a cry for attention, which will quickly run dry when everyone begins to realize that they will always refuse to enjoy themselves because being a sullen bitch gets them more attention.
A: Dude, last night's party was awesome! Except for John's girlfriend, she was being such a fucking cuntfortress the whole night and refused to have any fun no matter how many times we tried to include her. Why even bother going if you're going to be a fucking downer all night?