1) Buy them, steal them, build them, the point isn't that medieval folks couldn't do those things. The point is that they couldn't do those things anywhere near the magnitude/frequency as it is happening now in CK3.
Go open any military history book and see how many times medieval naval landing happened, versus good old marching over land to get to their target. Naval landings were, and frankly still are FUNDAMENTALLY riskier and difficult to pull off.
Why was Calais a strategically important city in 100 year war? Because it had a port that could support all that naval embark/disembark. Fact of the matter is that no army/nation in history, even in modern times can embark and disembark wherever and whenever they please especially in hostile territory. The only reason why people keep saying this is realistic or feasible is because they like the simplified game play(which isn't bad in itself, but the current implementation isn't good).
2) Obviously if I started as a duke and became the emperor, then for the majority of my game play my capital wasn't Constantinople. And for some reason you keep assuming that I am getting my butt kicked by AI. No. Like I said I am doing well. Never once have I let my capital be sieged down for a prolonged amount of time. AI is so predictable that all I need to do is wait by my capital and ambush enemy army when they make that ridiculous naval landing with huge disadvantage penalty. Which is a terrible move for them. This is clearly not a fun way to do warfare, and why the current naval system needs to change.
3) On the issue of terrain, it is obvious the devs wanted terrain and the consequent attrition system to mean more then it did in CK2, with all the terrain specific buildings like forest fortress, etc. Sadly, precisely because of the current naval system where enemy can just bypass all that with ships, all that terrain/attrition system is simply lost potential. Another reason why the current naval system needs to change.
4)Yes I hope we get updates on naval system
