I agree with the general assessment that fortifications should be difficult to take, especially without the correct equipment and preparations. Coastal fortifications should also definitely be impossible to take if you lack naval dominance, unless you assault assault the fort.
However, I disagree with sentiment that the Theodosian walls should just be another fort, for the first 100 years of the game. I don't think any other fortification in history has such an impressive track record of repelling sieges, and there's a reason for its reputation back then, and today. It was truly an engineering marvel, that drew on the resources of a continent spanning empire that's long since been reduced, and will soon have its borders limited to said perimeter wall. There's also reason the walls have never been replicated.
The scale of the walls was extremely impressive and would've been incredibly daunting for any besieger standing beneath it, especially one tasked with an assault. You had to cross a moat 10 m deep and 20 m wide before reaching the low walls, then you had to scale the 9 m tall outer wall, before finally reaching the 12 m tall and 5 m thick inner wall. This is all while taking fire from the 14 m and 20 m tall towers of the outer and inner wall, respectively. For comparison, most other medieval cities at the time only had a single stone wall, before cannons became prevalent.
I think the best way to translate this into the game is to give the Theodosian Walls a modifier that increases attrition and dramatically increases casualties from assault. These should scale with the level of maintenance, but obviously tick up and down, instead of being instant. To offset this, maintenance would obviously have to be more expensive than your average fort. Finally, a breach event should not be possible, unless you have cannons. Essentially, this would mean you'd need a large army with cannons (and naval dominance) to take the city.
It would also be cool if there was a decision to modify the walls for protection against artillery, possibly at the cost of a lot of money and perhaps the loss of the modifiers, essentially turning them into normal upgradable fortifications. You likely won't need it if you're big enough to discourage threats to the capital, but it would be nice to have the option. Perhaps a prestige modifier could be added to encourage you to keep the walls around in their base state?