Warrior Monks are decent units who perform adequately in both offensive and defensive combat. They also wear armour which protects them against ranged attacks. Warrior Monks are fairly fast in speed and are capable of weakening even an enemy Lord in enough numbers, although they are no match for Knights.
With adequate protection against ranged missiles by Archers, these units are capable of harassing the enemy by destroying buildings, capturing estates or other tasks which could endanger some of the less protected units.
Warrior Monks are excellent as a taskforce of their own and can be used to quickly capture ground or prevent enemy units from doing damage to your own economy for instance. They work suprisingly well in fighting against Berserkers, particularly in numbers. If you're struggling with a counter attack to Olaf, these work excellently. Note that no AI lord uses the Monastery, and therefore, they do not use Warrior Monks. Please also be aware that no AI Lord in the game recruits these units.
You can recruit Warrior Monks from the rank of Baron. You will need 1 gold as well as 10 Honour and 1 peasant from the campfire before you can recruit a Warrior Monk. Honour can be produced by the means shown in this article.