Located in the 99999th floor (it's a pretty god damn big building) of the Citadel, they guard the most important floor of the entire Citadel, and by extension the city itself.

They are droids, build for the specific purposes of surviving bombs, grenades, gunshots, rubble, geese, water, and up to 5 tanks! (the giant cannon shooting ones, not the people in large suits of armor) Their design is meant to be as "Screw off before I tear your legs off." as possible.

They apparently weigh over 99400 pounds and have the ability to get angry.
Nobody knows why, it's mainly a rumor, but it'd be kinda frightening knowing that war machines can get mad.

TRIVIA
The reason that they are robots is because,
1. No living entity could possible support the crushing level of armor plating and lack of cooling systems.
2. Screw you, robots are cool.
3. Sentient species can't be trusted, so they have robots instead.

4. Can't hack it, the wiring box inside is behind exactly 96 layers of who knows what.