Playing XL mod, Byzantines, early, hard.

When playing a faction that I know will be in the front line when the Golden Horde shows up, I use a tried and trusted method of dealing with them - upgrading the defences of the province or provinces most likely to be hit, garrisoning it/them with hand-picked troops, then withdrawing all other units.

The Horde invariably assaults the castle and even if they succeed in taking it, they've lost so many men and are so weakened that they are fairly easy to deal with thereafter. Often they fail in the assault, losing perhaps 2,000 men in the process.

However, in my current game, my first using the XL mod, things went very differently. First off, the Mongols had a lot more mortars than I've noticed before - but that's OK, that could happen. But also, they just ween't taking many casualties from the castle defences. This was in Georgia, it was a Citadel with ballista towers, and the damage seemed far less than ordinary arrow towers. I can't remember if I've ever managed to develop a province to this stage before when facing the Horde, so I don't know if ballista towers are simply not as effective as arrow towers anyway (though since it's an upgrade they should be) or whether this is a feature of the XL mod.

So the Mongols knocked down my outer wall, at which point I withdrew all my forces to the inner bailey. Thereupon, the Mongols came in through the outer gate, even though it hadn't been breached (presumably the mechanism is now assumed to be in the enemy's hands, but at that point they had no troops inside the castle). The Mongols now infested the outer bailey, and although I had missile troops inside, they took HARDLY ANY CASUALTIES either from my archers or the castle defences, while the Mongol archers shot my troops to oblivion! The result was an ignoble defeat, the loss of Georgia (and some elite troops) and absolutely no dent in the size of the Horde.

Another thing I noticed was that the Mongols were speedily destroying my huge inner stone walls just by firing arrows at them, which is hardly realistic!

So, what gives? Should I not have upgraded to ballista towers? Or are there some tweaks in the XL mod that causes this? You don't get that many assaults against well-developed castles in a game, so this may just be something I've never seen before.

On a related note, what determines how long a castle can hold out during a seige, food-wise? If you stick a decent-sized garrison in a castle, it never seems able to hold out more than a year or two.