Below are the different strategies against the Sultan.
Not much defense is needed against the Sultan's army. His large groups of slingers, which target workers on farms, quarries and mines as well as woodcutters may prove an annoyance, but a few small groups of archers or horse archers scattered around your industries should be able to deal with them. With regards to his main attacks, the Arab bowmen he sends can be killed easily. Arabian swordsmen may prove more troublesome, but a decent number of crossbowmen may handle them very well (a swordsman needs 3-4 crossbow bolts to fall). Swordsmen will be busy destroying walls (which is done very slowly), making them further vulnerable to projectiles. Needless to say, the Sultan's first rush is the most devastating out of all his attacks, unless he is doing particularly well and has built up a large army.
Like the Rat, the Sultan can be dispatched with anything. He only has Arab bowmen on the keep, so you can kill the slingers out of their range with horse archers, normal archers or crossbowmen. All the slingers need to be killed, even those behind the keep, as slingers can deal a lot of damage to melee troops. Then you can either scale the walls without capturing the gatehouse (although marching swordsmen may attempt to advance via the stairs) or pick off troops with fire ballistae or crossbowmen. When most swordsmen are removed, move in for the kill.