So I'm stuck at the part where it says to find the north bridge control mechanism.

[TigerKneeCrush]

Well when I get to it and press check it says it's the lever for the north gate. You don't need to use this now.

 

[Warshade]

There is 5 towers a north east a north west a south east and south west and a central. You need to go to the bottom of the north east tower and raise the draw bridge then the bottom of the south east to lower the south draw bridge then the top of the south west to raise the gate so your forces can get in. Somewhere in there you need to go to the central tower and get the key too. The story will tell you that part though. It is a bit of a maze for sure.