1. We don't know the shape of Erf, much less its size.
2. I'd bet good money that twolls can go into tunnels, much like most humanoid units probably can; I mean, if mounts like spidews and sourmanders can go into tunnels.
- I doubt a twoll is nearly as powerful as even a weak dwagon.
3. Ports can lie on lakes and on rivers. I see no reason, however, why there couldn't be seas on Erf. Though we don't know much of Erf's topology, a sea is just a lake that doesn't have any outflowing rivers.
4. All water probably has the same effects as far as defenses go. I'm sure that there must be a way to mount an assault by sea, making a moat or a lave lake a safer situation than being surrounded by a river or lake that might allow the practical execution of an attack using larger watercraft.
5. We know that not all dwagons are created equal: not only do they seem to have different attacks, but we know that some are "stronger" or "weaker" than others of their same kind. Ansom clearly sayss: "These may be weaker dwagon types, but they are strong examples."http://www.erfworld.com/book-1-archive/?px=%2F058.jpg