I believe you need to take the key from a knocked down guard near one of the gates.
Or knock down the gate, I don't remember.
Well, what happened was:
I couldn't get the guy behind the counter between the two gates to acknowledge me, so I had to TCL on over to him. Then I couldn't get the guard to acknowledge me, so I had to TCL on over again.
Then the fight started, and I totally didn't think of looting a guard. I'll give it another go, because it is a nice change of pace.
Also loving the OP's mod list. Not using all of them, but he's got a pretty nice selection there.