Decent siege defenses are rare in my experience as the AI will often wait 2 years for your garrisson to starve rather than assault. If you are defending with a small garrisson it will buy you considerably more time, but the AI is more likely to assault and easily defeat your undersized force. I prefer to use Chivalric Foot Knights and Chivalric Sergeants to defend. The CS I use as gap pluggers and the CFK's as flankers/anti-cav. Some arbs/crossbows can be useful as well positioned behind the gatehouse shooting through at the enemy. As the Muslim factions I send to go Saracen and missile heavy with some Ghazis.