24 str + 8 power draw + masterwork war bow + 3 large bags of khergit arrows.
Thats my siege defense. Every time a friendly archer dies or I start to get low on arrows I pick up more. I was invaded by the Vaegirs and used their own arrows against them, pulling them out of the walls and shooting them back. With that bow you can kill just about anything in 1 shot. The only things that can occasionally survive an arrow from it are the particularly beefy enemy lords, and even then if they are slightly injured before the battle they drop in one shot.
I won a fight with 2200 Vaegirs vs my garrison of 200 assorted Nords (30 Huscarls, 30 Veterans, 40 warriors, 40 trained footmen, 60 veteran archers) at Dhirim with full difficulty settings and battle size on 100 (my computer just can't handle 150 on Warband, it could on normal M&B but not anymore). I kept taking out the guys going to push the siege tower and picking off the guys without their shields up. By the time the siege tower was at the wall I had probably killed 100 men and had full arrows from looting a few fallen veteran archers. My little group of huscarls that spawned did a fine job of holding the line on the siege tower and I stood on the side dropping the enemy as fast as I could loose arrows, focusing on the particularly beefy ones to keep them from breaking through. I was standing on a pile of dead Veteran archers so refilling my ammo was quick, and I killed at least 150 more right there. It ended with me looting a shield and axe from a fallen Huscarl and falling back inside the walls with my reinforcements and taking out the final wave in melee. The result was about 350 dead on their side and only 30 or so on my side, mostly archers since they were getting sniped off the walls by the Vaegir marksmen.
The masterwork war bow is so overpowered. When facing enemy cavalry I just get my infantry to form a tight line and get at the back of them, warbow in hand. The knights charge in and I just aim up and drop one after another after another firing over my troops' heads as they ride up and try to hit my infantry. The cavalry charge always ends with a bunch of headshotted knights and a bunch of loose horses.
It works great on siege offense too. I can just stand near the wall picking off archers every time they stick their head out and picking up arrows from the ground around me when I get low.