Indeed it is open. All the vanilla and BI ones are within the building I think but they work well wherever you put them. Also the ground is elevated in the map when in game so the ground does come up to the bottom of the tunnel. The main problem with having the tunnel open I believe is that you cannot fight on the tunnel and so people worryingly run up and down oblivious to the world. This may seem like nothing but if you fight around it you end up with a fight at each end and troops running all over the place to get to the fight at the other end - basically it doesn't look too real. I think the right effect can be made by a platform without any slots - like on bridges if I'm correct - where you cannot move onto the bridge but can move and fight on it to get to the other side though I haven't tried that yet.
When fiddling around with things I was actually rather impressed at how flexible the game engine is with the entry and exit to tunnels. The troops are able to actually "jump" about their own height to get onto a tunnel and get off again. I'd advise some experimentation to find out what geometries are suitable for what purposes.
You mentioned some other questions?
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