If you do some research, you'll find there's a weird number of people battling snails from medieval times. I compiled a few I found but there's many more.
Why is this? Did snails have some kind of significance?
We don't know. Seriously. There are as many explanations as there are scholars.
Medieval people thought it was weird and funny, too. They even parodied it.
The British Library's Medieval Manuscripts blog, which I will shill for every chance I get, has some more great examples here.
According to the late, great art historian Michael Camille (in his book "Image on the Edge"), there have been a number of theories behind the iconographic motif of the snail in 13th, 14th, and early 15th century marginalia. Citing other historians and art historians, he mentions parallels with the story of the Raising of Lazarus, individuals cloistered away in the safety of fortified castles, the Lombards, social climbers, and more lewd, sexual interpretations. For more, see "Image on the Edge", pp. 31-36.
(as an aside, the snail motif appears in late 15th century 'trompe l'oeil' marginalia, but Camille dismisses this era of medieval illuminated margins as "The End of the Edge" in the last chapter his book--one of my quibbles with his work).
