Damn, that gate looks as if it might have a security detail too.
Then again, if there isn't any activity in building houses, I wonder if there is any more open way around the back, because that steel bridge over the river looks as if it may provide a way around- unless there is also some sort of security detail on that too. But looking at it on a map, I wouldn't imagine a treeline along an old abandoned mill to be impregnable.