This is like inviting a friendly knight to try and break into your castle to find weak spots in the walls or gates. The good part is they can find a lot of different ways an enemy could get in, even through tiny cracks you might not have noticed. The advice they give you is super specific to your castle. But, if the knight isn't careful, they might accidentally cause some damage while testing. Plus, you really have to trust them not to turn against you. Also, if you only pretend to defend the castle when they attack, it might look stronger than it really is.
Think of this like sending a bunch of carrier pigeons to fly around your castle looking for weak spots. They're great because they can do this quickly and often, helping you catch problems before they become big. They can even help you follow the kingdom's rules on keeping castles safe. But sometimes, the pigeons might get confused and report a spot as weak when it's actually fine, or they might miss some spots because they're too hard to see.
This is when you have a group of expert knights pretend to be the enemy and really try to break into your castle, just like in a real attack. It's a good way to train your guards and see how well they can defend against smart intruders. But, it's expensive, and they might not find every single weak spot, just the ones they need to get through.
This tests how easily someone could trick the people inside your castle into opening the gate for them, using lies or tricks. It's really important because this happens a lot. It helps your people be better at spotting tricks, even in their own lives. But, it only looks at trickery, not other ways intruders might try to get in. And since the tricks change all the time, you have to keep testing.