Here's an overview of the units specific to Stronghold:Crusader
as written up by Ztolk.
You can find the other units here
With special thanks to Jeru and Ztolk who liberated the images.
More powerful than his European counterpart, while maintaining the same speed, rate of fire, and range, this archer is great for defending your walls and towers. They work in the exact same way as the European archer, but are stronger and do not require any weapons to manufacture. A few towers full of these guys will send a powerful army to their sandy grave.
Cost: 75 gold
These shirtless torchbearers make that pathetic spearman from the original Stronghold look like Superman. They are the weakest unit in the game. Practically useless for fighting, their strength is used inside the enemy town. Each one of these little guys can set a building on fire. Make enough of them, and your enemies entire castle turns to ash. They do have the nasty habit of setting themselves on fire, so make sure you have a lot of them. These are also the only arab unit that can dig a moat.
Cost: 5 gold
Similar to the slave, these guys can barely fight. They throw a small rock out of a sling towards the enemy. Although it worked for King David, they barely do any damage in Crusader. If massed, they are best used for protecting other units, taking out your enemy's civilians, and manning walls, towers, and gates.
Cost: 12 gold
Assassins are the most unusual unit in the game. They are like a combination of ladderman and pikeman. They walk to the enemy's wall under cover of darkness, throw their grappling hook onto the ledge, and climb up. Once on top, they can walk on top of gates to open them for your ground troops. They are also fairly resistant to archer fire and have a nasty sword in their sheath which will make quick work of the defending archers. Best of all, or worst of all if your enemy uses them, they can not be seen until they attack, which is often too late.
Cost: 60 gold
These speedy bowman are tied with knights as the fastest unit in the game, and are great for many uses. They can shoot while moving, so are great for patrolling an area. Their quick rate of fire quickly eliminates lightly or unarmoured enemies, and in large numbers, it's almost impossible to even get near these guys.
Cost: 60 Gold
Sacrificing strength for the speed advantage over the European Swordsman, the Arab Swordsman is the melee force of the arab and mercenary armies. Only swordsman, knights, and the Lord are stronger, and large numbers of these will tear through wall and flesh alike in order to vanquish the enemy and their lord.
Cost: 80 Gold
Another odd unit, the fire thrower does what it says, throws balls of fire at enemy buildings and units. They are great for taking out enemy towns, with the ability to set them ablaze in seconds. They also do serious damage on the battlefield, and can turn many a unit into a burning corpse before being taken down. Be wary when defending your walls with them, however, because if the enemy breaches them, the fire throwers will continue raining flames upon their own castle.
Cost: 100 gold
The giant bow that saved your castle so many times in Stronghold is now available in a portable form. Once assembled, the fire ballista can move anywhere on the ground, and when in near enemy buildings, it can send blazing bolts into them, lighting them on fire. They are also good for defending the home front or battlefield. Players have lost entire armies of mighty swordsman trying to reach the ballistae that were tearing them apart.
Cost: 150 gold, 2 engineers
Trained at the Cathedral, these slow moving clergymen pack a decent punch and can take more arrows than their Stronghold counterpart. Probably too slow for attacking, you can use them as a cheap source of castle defense.
Cost: 10 gold
* denotes a former staff member.