>Limbo can be reached on any level of dreaming, it does not
>matter how deep you are but rather if you are aware that it is
>a dream. Further, limbo is its own separate layer with
>surroundings and rate of time unique to limbo.

It's a shared subconscious and the time just moves even slower (I think Leo explains this when he tells everyone they can't die or else they'll go crazy because they'll be stuck in limbo for so long.)

I don't think it matters whether you know you are dreaming or not. Saito knew he was dreaming but still went there when he died in the dream. Apparently you get your brain wiped or something when you head into limbo and think that it's the real world even though you can build shit with your mind. And we have no idea how Mal and Cobbs ended up there in the beginning; it's never explained.