One thing that bugs me about the game is that the "feudal" period is so short. You as a human player tend to fight with militia/mailed knights for a very long time, with feudal knights for a very short time (growth from fortress to citadel is rather fast) and then fight with top grade troops for a very long time. The Feudal knights that supposedly dominate the battlefield for most of the period are rarely seen, particularly not in the hands of the AI that is so busy recruiting crap early on that it wont have money left to field better armies after upgrading settlements. My remedy is to mod castles so that they get feudal (but not dismounted) knights one tier earlier than in vanilla, and it helps somewhat...

In vanilla I've basically had four enemies sending decent armies against me. The portuguese sending dismounted portuguese knights against Caernarvon (that this invasion doesnt make much sense is another matter), the English defending with armoured swordsmen at Caen/Nottingham against my Scottish/French, and the Hungarians sending tonnes of dismounted feudal knights against Sofia (probably recruited at Bran). Finally the danes tend to send a lot of dismounted huscarls (available early) which can be tough to beat with low grade units.