As highlighted in issue
#1018, the current physics API isn't exactly nice. It's not the worst, but Fixtures especially are pains to deal with. I understand why Fixtures can be nice to have, but in most of the code I've written or read, one Shape becomes one Fixture, is attached to one Body.
Ideally we'd have some api that kind of matches the love "Hello world" feel, it's just a few lines to do the simple things, but if you want to, you can get a more advanced api. In this case, if you want a circle that has collisions, you'd probably want to be able to quickly create a Body, a Circle shape and the corresponding Fixture, then attach that Fixture to the Body, ideally using only one or two functions.