Although they might not have any prerequisites, level 2 or 3 subjects (Arts ones at least) generally assume you're pretty well versed in writing essays and won't offer you as much guidance as the level 1 subjects.
I know I almost considered doing Age of Stalin (Level 3 History subject) in my first semester of uni. It had no prerequisites or recommended background knowledge, but I was strongly advised not to take it since I had never written a history essay before, and by taking a level 3 subject, the assessors would have assumed that you'd be quite familiar with the structure or whatever else you'd need to employ, as well as research techniques, to write a decent essay for that discipline, so they'd be much harsher in marking you.