Actually, all the reasons you listed are poor in terms of the town's setting. Being so far from neighbors, having only one entrance and exit, among other things, sort of remind me of what happened at Dunkirk in WWII....IDK, just a thought.
Yes you're most likely right, but I think historically Coastal ports tend to fall to superior naval forces than land based seiges.
Before the great wars that is and the invention of blitzkrieg style tactics.
Really though I don't think any city not even the IC is truely the strongest, as everyone of them is liable to invasion of one sort or another it seems.
Also of note is the fact that most seige engines would have to be shipped in due to the lack of trees.
Anvils on the bottom of a hill haha. Just bring in a catapult or two and its toast. Id take the DB in Cheydinhal over the Thieves Guild in Anvil anyday
Just to prove my point ( sorry Rainman nothing personal intended )..
Cheydinhal is ringed by hills and mountain ranges, due to the being on the far side of access to the IC is isolated from all its neighbours.
Bruma is closest on a direct path, which however is more of a pilgrims track in most cases, and goes through mountainous bandit / nord, ogre, daedra, and minotour territory.
Its surround by cave systems is run by an inefficent goverment, the landscape suggests to me to be prime seige potential with trees, good ground and high vantage points.
The Dark brotherhood is a small group dealing in hired kills with no personnal loyalty to the county let alone the country.
Supplies, spying and infomation is not in its preserve, even if they did not just torch their HQ and cut.
Finally there is absolutly nowhere to run to if the walls fall, the castle is about the only place and is quite exposed itself being looked down on from all sides.