Middle Ages Art Depicts Snails To Be A Major Threat Apparently

If you were to look at art from the Middle Ages, you might find yourself staring at some knights in a fierce battle with ...uh... snails. 

Why snails? That is a completely mystery. There are some theories Digital Medievalist, Lisa Spangenberg says that "the armored snail fighting the armored knight is a reminder of the inevitability of death," a sentiment captured in Psalm 58 of the bible: "Like a snail that melteth away into slime, they shall be taken away; like a dead-born child, they shall not see the sun."

What do you think it could mean?