You can play whatever you want! If a character is too powerful, the admins will usually tell you if you play anything that's out of bounds. Usually it's just God characters that get the boot, or main characters ripped from canon sources, like the Doctor, Michael Weston, Etc, Etc.
It's about...well middle medieval xD There are guns, but those are limited only to Connlaoth, and they're not very awesome guns either.
Nope! No main plot to the story! It's just a big ol' world doing whatever. You can of course make your own plots if you so want and try to get people to join in, just no guarantees. Big world altering plots do need to be approved by the admins though!