I had started to write some of this up to post in the balance poll that was just posted, but figured this would be better for more detailed discussion. I know much of this has already been stated but I think I can offer some details other were asking for - particularly in regards to garrison mechanics.
Here are the main points that summarize the issue from my perspective:
- Naval melee units tend to bring a large power spike in combat strength, far above that of most land units of the same era
- Defensive buildings can not raise the combat strength of cities to compete because they do not stack with garrison unit strength
- Naval melee units suffer a 50% penalty in regards to garrison strength (intended?), which prevents them from otherwise offering a solution
- HP recovery of cities is very slow
- Defense process halves the ranged combat strength of cities - bug?
- Naval melee units can attack forts, often making them a liability rather than a potential solution
- The ironclads 33% bonus vs cities, and naval siege/vanguard promotions, exacerbate all of the above
So a very typical scenario when ironclads come around is that you have a city with walls/castle/garrisoned unit, but the limiting factor for combat strength is the garrisoned unit. Lets say you have a musketman garrisoned, the combat strength of the city will be 30 - matching the ranged strength of the musketman. Even if you have an arsenal at this point, the CS will still be
30. Garrison an ironclad yourself? The CS is still 60 * 0.5 =
30.
Whereas the ironclad has base 60 *1.33 bonus vs cities = 80. Typically the ironclad will have at least a few promotions that will increase the combat strength by another 20-45%, even as high as 95 - 130% with just one of naval siege or vanguard. The actual combat strength of ironclads vs cities ends up being anywhere from
100-250.
In general this results in coastal cities being a massive liability on higher difficulties. However the player can also use this to their own advantage in the right circumstances. In my current game, an unprotected channel allowed me to waltz through the Netherlands' empire taking cities in one or two turns with a single naval siege/vanguard ironclad and a few supporting units (most of which were just an afterthought). I ended up taking six cities including the capital in a handful of turns.