You should use most of your money on horse archers, and when you have about 80-90, attack the Sultan and spend the remainder of your money on assassins. Kill the Sultan and with your new money, attack Saladin. Buy more horse archers and assassins, and Saladin will fall.
(Alternative, possibly faster way.)
This must be done quickly.
Build granary, 3 armories, market, barracks, and mercenary post. Buy 75 leather armors, and 75 crossbows. Train 75 crossbowmen. Send the crossbowmen outside the Sultan's northeast wall, and pelt his troops with bolts. As the crossbowmen are softening the Sultan's forces, with the remainder of your gold buy assassins. Once you have spent all your gold on assassins, send them all straight for the Sultan. After sending these assassins to Sultan, sell all your goods and keep training assassins, around this time you will be granted gold for killing the Sultan, allowing you to train more assassins for your second contingent. Send the remaining crossbowmen just outside Saladin's walls, making sure that no towers block your line of fire on the keep. Group all of your assassins out of range and wait until your crossbowmen have killed most of the tower archers and keep guards. Next, send your assassins straight for Saladin and he will beg for mercy.