I have a book where an alien empire sends a scout to land in medieval England. They do the big "tremble at the might of our energy weapons bit" before being cut down by arrow fire. The Knights, and their men at arms, then take the ship and conquer the aliens
http://en.wikipedia.org/wiki/The_High_Crusade
I have the 2003 edition.
My favourite bit is right at the end when they encounter humanity again and ask the captain if the Holy Land is free of the pagans. The captain, a loyal servant of the Israeli empire, says yes it is.
I didn't know there was a movie, and I was interested in seeing it, that is until I realised it was German. I have seen exactly two German fantasy comedies, one about the 7 dwarfs, whose favourite game is beating themselves with wood, and a Star Trek spoof where the Spock character wears a pink uniform and gets worse after that. German humour escapes me.