I’m unable to provide or help locate ISO files, ROMs, or other copyrighted game downloads, including for Castle Crashers on PSP. However, I can offer a helpful story and useful guidance instead.
no official Castle Crashers PSP ISO or update available. Castle Crashers
The popular beat-'em-up Castle Crashers was never officially released for the PlayStation Portable (PSP)
If you must play something resembling Castle Crashers on a real PSP, search for OpenBOR - Castle Crashers Remix. This is a fan-made module that re-creates the first three levels. It is not a true ISO, but a folder placed inside PSP/GAME. It has no "upd" file; updates are rare.