U have to go all the way around the opposite side.
youll find a gas filled area.
finding this will add an operation to the war table in which you can have a bridge built.
build it to cross over.
Around this area is a keep you must also capture.
capturing the keep adds a quest involving darkspawn.
pursuing this quest leads you into a mini dungeon that acts as a shortcut to the opposite side of the gate.
right inside the gate is a lever you can pull to open it.
done.
