Technically the only reference you need is the film, but if an outside source would help the essay sound better than please do so. Paper should be four pages, have a clear thesis, avoid excessive plot summary, and be well-edited. The topic of the essay is really up to the writer. But here are some examples of topics:
1). Monty Python’s characteristic cartoon artwork from the TV show, as crossed and combined with medieval manuscript illustration (complete with a grumpy monk).
2). the English view of the French (and there really was a long history of entanglements between the French and English crowds in the late medieval period, and constant conflict about land and sovereignty).
3). techniques of medieval siege and defense (catapulting dead animals really did happen, but it was less intended to land right on top of those laying siege than to spread disease among them).
4). how the film parodies the Trojan horse legend.
5). how Robin and his minstrels parody the traditional relationship between a hero and his bards
6). how the running jokes (coconuts, the ever-analytical peasant Dennis) build up in humor over the course of the film.
7). how the Zoot scene parodies Galhad’s legendary sexual chastity.