Thieves are ingenious units in Stronghold 2. These units have the ability to steal from the enemy Treasury if they can reach it. Not only this, but Thieves will also appear to the enemy disguised as one of their own Farmers. There is however a subtle way of telling whether your own Farmers are enemy thieves of course.
Simply left click, drag and attempt to select them. If a unit health bar appears over any of your Farmers, they are in fact Thieves in disguise. Send in some of your troops, and you will be able to kill them off easily. Your ranged units may also uncover the true identity of a Thief as and when the Thieves get closer to them. In addition to this, your Guard Posts for catching Criminals can also uncover enemy Thieves as well.
To steal from an enemy Treasury, the path must already be accessible by your units. If it is not you may want to consider some kind of Assassin to capture a Gatehouse and allow your Thieves entry into an opponents Castle.
Note however that any man traps will still be triggered by your Thieves moving over them, thus killing your Thieves. Once inside the area with an enemy Treasury, simply select your Thief and left click on the enemy Treasury, which should turn red. Your Thief will now move over and should automatically start stealing on your behalf. Patrolling enemy guards can and will eventually notice your Thief stealing.
In actual combat however, the Thief is fairly weak at both offensive and defensive attacks. He can hold his own ground for a while, but will very quickly fall under ranged attacks. His only real use comes from the ability to steal from enemy Treasuries. 25 gold at any one time will be stolen at regular intervals by your Thief. Money stolen will automatically and instantly be added into your Treasury. Please also be aware that no AI Lord in the game recruits these units.
You can recruit Thieves from the rank of Earl. You will need 10 gold as well as 50 Honour and 1 peasant from the campfire before you can recruit a Thief. Honour can be produced by the means shown in this article.