Assassins are exceptional covert warriors in the game that can easily help to bring you victory, although you pay for it in Honour. Assassins will appear completely invisible to all enemy units until they are essentially on top of them or are engaging in direct combat, and it is too late to do anything about.
They can also scale walls with a grappling hook, meaning that they are the ideal unit to capture any Gatehouses in your path. They are fast moving units that can quickly get to where you sent them.
Assassins are extremely useful as harrassment units, for capturing estates, or for destroying enemy buildings. One useful tactic is to send them into a poorly guarded area of an enemy castle, have them scale over the wall and then deal out as much damage as possible.
It's worth noting that while Assassins are invisible troops, traps and other defence systems do still affect them, regardless of their visibility. Make sure you keep this in mind and manouver your Assassins around these systems where possible. Please also be aware that no AI Lord in the game recruits these units.
You can recruit an Assassin from the highest rank of Duke, probably to reflect their superior covert abilities to any other unit. You will need 100 gold as well as 200 Honour, and 1 peasant from the campfire before you can recruit an Assassin. Honour can be produced by the means shown in this article.