The biggest ive been hit by was 3k plus. Damn siege lasted a good 30-45 mins. I also have a huge battle size adjusted.

2.1k sigeing my 800. We won

Its highly unlikely that they would besiege you with over 1k in a castle at once, but I've had 6k attack me (It was an invasion army ambush). Luckily, I was nearby with butt-ton of my op af Archer Knights (my CKO), so I held them off with barely any casualties, most of which were caused by the demons (There were at least 200)

5 thousand, from Eyegrim the Devourer.

Your defending strat is not going to work very well. If you load it with actually good troops, it'll run a bazillion denars a paycheck for a lot of guys who aren't doing anything.

The strongest defense strategy is to lead the marshal host to the siege of your friendly fief, then wait for the enemy to actually assault it. Their whole field army will pile in. Then you pounce with yours (obviously you must intervene faster than the enemy can wipe the garrison). Your whole host will join the defense and, unless you're inexplicably without a good bow or crossbow or you're still ridiculously outgunned, crush the enemy invasion.

With their field army thus destroyed and your own mostly intact, you can reverse the polarity of the war and go on an unchallenged offensive. This should buy you enough time to seize 1-3 fiefs before the enemy can retake the initiative.

I recommend for this a garrison of 200-300 with a comp of 40% ranged and 60% infantry, with these being high grade commoner troops. Everything else (rare and noble troops, all cavalry) you're going to want to conserve for decisive field battle and assaults on enemy fiefs.

Eyegrim only assaults towns and even if he wins, the town just gets sacked, so I wouldn't bother worrying about defending against him. What I ended up doing was just putting 1500+ 1 Denar troops in my castle which served as my capital. I had a hundred or two top tier troops, but a bunch of them were just there waiting temporarily if I wanted to change the composition of my field army. I'm not sure exactly how many troops you need for the AI to guarantee never attack based on the coding, but nobody ever besieged that castle. Even 2K 1 Denar troops are pretty affordable. My understanding is that it's just the number of troops and not the quality that determines the AI behavior for selecting targets.

In PoP it is actually quality x quantity (e.g. 1x Legionnair = 10x Peasants).