The problem I was having with having levels was that the boys would pee and mark only on other levels and not at the bottom of the dang cage. So I would have to clean each level everyday becuase they would soak it.
if you place the hammocks stagtegically and maybe get a couple hidy-holes (hamster balls, organizing bins, etc.) to hang up as well i don't see the need for solid levels. the levels themselves are meant to give the rats more room to run and jump and for you to be able to put igloos and the like on. but if you have enough hammocks and can tie things up this will serve the same purpose. with a bit of ingenuity and a bunch of zip ties you should be able to get away without platform levels.