The problem is the first step up from the middle level. It wouldn't be easy to put a gate in front of the bottom step - there's nothing to fix to on the bannister side.
We currently have open stairs (i.e. no risers) on our staircase, in a 3 story townhouse. I have a soon to be crawling baby, and the gaps between stairs are quite large (around 15cm), easily big enough for the baby fall down from the middle level.
I've been looking for some kind of rod...