A knight assembles a clandestine operation in order to rescue the princess of Jerusalem from the infamous king during the crusades.

I think his logline is about the plan more than the action of rescuing. I think It could be like this: ...during the crusades, a knight must rescue a princess to save her kingdom before they kill her.
Because the plan could be shown in a scene, however it is not about the plan or yes?
(sorry if my english is bad)

Is this based (less or more) on an actual historical characters and events?

Is the story about how the knights assemble? If that's what the story is about, then your are good.

If the story is about the actual rescue operation I would drop the part about the assembly and get on with what they must actually do to save the princess.

"When the princess of Jerusalem is captured, her betrothed and a group of knights must..."