Tactics for Fortress against Tower on an XL map.

I'm definitely not an expert on the Stronghold, but from your post, it's clear that I can teach you a thing or two. First, what do you mean by "not enough flyers"? Yes, there's only one, but what prevents you from detaching single units and using them to block until the main stacks arrive? Second, I understand you're letting the computer develop up to Titans? Why? Even on an XL map, that's strange... Third, if you're afraid of magic, look for a countermeasure (either artifacts that limit/ban it, or gather more melee units for mass haste). And fourth, ditch that Ship and switch to the Knight, and go online instead of playing against the computer.