Slitherine

When playing against the AI on various difficulties, what do you feel is a reasonable garrison for forts of various levels? Something that can at least deter the AI from assaults and hold them up a few turns. Conversely, what is the minimum force you are comfortable with to assault a fort, at various occupancy, levels etc.

I've never looked at it too closely but I don't put too much trust in Level 1 except as a method to stop the enemy advancing more than 1 province in a turn (if I could, I would build 1 palisade in every province...). Level 2 forts without garrisons/palace guards can be assaulted (don't know if the AI does it but 6 mercenary + 6 archers seems to give fairly good odds) by a not too big army. I usually siege a level 3 fort.

Level 4 is the real monster in my opinion, Babylon and Rhodes especially. You can take them but I had to recruit so many extra medium foot for the siege/get navies to supply an army. Then again I may just be doing it wrong, perhaps you can assault them quickly? Tell me what your experience is.

I tend to do similarly to you, but whether I assault or siege is usually driven by 2 factors;
1) Whether I can accept losing some units. I tend to try and never lose units as getting good experienced units, which are really useful, is a real priority in my games. Small quantity will generally beat large poorer armies (which is the opposite of FOG2).
2) Whether I have the time to siege or whether I'm in a race for some other strategic point.

The problem I've found with fortifications and the siege rules is that by mid-game both the AI and I are marching around with armies with 100+ normal infantry, and these armies seem to squish even the best fortified cities. I'm not 100% certain how the siege calculations work, but there doesn't seem to be a limit - or much of a limit - to the benefit of bringing lots of "besieger" units with your army.

Despite this, I still build every level of fort I can, whenever it's offered. Another challenge the game has is that a couple of guys without even a General can sneak through your lines and flip the ownership of your territories. On another thread, someone pointed out that this forces WWI-like tactics of maintaining a solid line of armies, but even then sometimes a small army can march into a territory, get an indecisive result in the first battle, and hen continue on through and seize a territory. Having big Forts everywhere - not just in border regions - seems the only way to prevent continual mopping up actions from these nuisance armies.