How many castles do you need?

[katank]

There isn't a set number. Really depends on your faction. Italian factions, Egypt, and the Turks can get by with very few castles (2 or 3 in a large empire of 30+ provinces).

For other factions, I'd say anywhere from 1/7 to 1/4 of your total territories. Specialize them for troop production (infantry, ranged, cavalry etc.)

As for the defense bit, I think that huge cities are actually better right now due to the fact that a city's cannon towers aren't bugged. Citadel's cannon towers only shoot ballista bolts.

[PaulTa]

Well it definitely is faction specific, since some factions make most of their best units from cities (Spain, for instance, get's gendarmes, tercio pikes, and musketeers from cities).

Generally, I see it like this. The only economic bonus that a city has over a castle is increased trade. If the city is constantly under siege, then the trade bonus is null anyways. Since the settlement is constantly under siege, that means that it's in my war zone, so I'll lean towards castles.

Then again, there is an exception to that as well. Occasionally it's nice to have a castle on an island or by water for quick sea based reinforcement. In some cases, border cities generate way too many florins to be converted in to castles (like vienna for germany early on).

[Redtroll]

Until now I've played France and i'm in the middle of a byzantine campaign. Both countries rely heavily on Cavalry troops - only available in castles (less true in late game as french military academy allows you Gendarme and mounted bowmen - silly btw, they used hoses only for strategic moves, never fought like huns or mongol. Scott guards are good too).
City troops are largely worthless out of walls, so you have to keep several castles if you want to retrain your army easily.
As for the mongol (and later timurid) threat (trough I've won just before they came for me), castle are a must to repel the nomads, due to their several ring of walls.
As French, I've kept 1 castle per front (Spain, Denmark, Italy + 1 facing England -"perfide Albion"), and several in holy land
As master of Constantinople, I've got no less than 4 castles against the eastern invaders (Tbilissi / Cesaree (for high level troop production) / Adana + just took Acre (alone in a sea of muslims ...)
West is more peaceful with Bran and Duras as recruiting centers. Palermo and the castle south of Kiev have just been captured too, and will be made town when all population will be faithful Orthodox and less prone to revolts ...
So I'm certainly a "castle heavy" player, but my empires nevertheless give me profits every turns.

[Musashi]

Heh, I'm playing Byzantium and I've managed nicely with only one castle (Corinth) for ages... I managed to take everything from Vienna, Zagreb, and Ragusa in the west, to Trebizond in the east before I finally got another castle (Because the turks begged for a ceasefire and I agreed on the condition they give me Caesarea, so I just left it as a castle).

I even made Venice a vassal.. And then took Bologna from the backstabbing HRE and gave it to the Venetians, since they were down to their last province when they finally accepted vassalage.

I just did a lot of combining units and only a little retraining. Plus I used spears from my cities, and those took most of the casualties.

[Skott]

So far the heaviest ratio I have done is probably 1:1. Like mentioned above the faction position you start with can determine your needs. I try to keep more cities than castles to keep a better flow of money coming into my coffers. I try to keep my castles on the front line and all cities behind them. In some cases one castle per modern country boundaries is a good way to go. This is easier to do once you have expanded and pushed your boundaries farther and farther out.

[baron_Leo]

In my Portugese campaign I had a castle in the Iberian peninsula and Tolouse. As Western European states only attacked Tolouse (if they would go south they find 2 not really defended cities, but he just keeps on attacking Toluse with full stack army, with all the units XP maxed out because of the numerous fights), it was more then enough.

[lobo76]

I am turk now, and am quite castle heavy. Reason is to produce the ottoman infantry. They are good missile troop, and pretty decent melee as well. (11 attack and 18 defense). My all purpose troops.

For borders cities, well, i prefer them to be CITY instead of castles. Sure, they only have one wall, but once you managed to increase them to HUGE city, they come with CANNON towers (gunpower period or not). Absolutely devastating, have incredible range and Unlimited ammo. When and if any siege occurs, I just need to sit back in my city while the cannon tower lay waste to the attackers. The siege troops only stay out of (arrow?) tower range, but the cannon tower range reaches them easily.