I like what you came up with.
The swamp needs to be lowered down to the shallows area.
We can nix the tunnel, and have a couple of underground passages from the back of the hall, that one way attack the swamp and somewhere on the right of the map. These one way attacks are depicted by a cave entrance, but the tunnel itself is not shown.
You seem to be the only one to come up with a GP layout, so I am just going to go ahead and offer you the rights for this map. If you go and read the link that I provided to the original Siege map, you will see that numerous people have asked for a Siege II. That along with mibi's blessing, gives us enough to submit a design brief. If you go ahead and do that, and it gets aproved and moved into the GP workshop, I will be available to continue on as the Artist.
One thing that I would suggest though, Reduce the amount of territs a bit. We want to make sure to have enough room, and all of the territs need to be large enough so as to be able to be legible once we scale the map down for the small version.