Note: it looks like there is no way around splitting up the pillars / walls of the gate into several tile-width (70*36px) pieces. Blocking can be done by adding invisible blockers but without splitting up the pillars into several smaller pieces, we'll always run into z-ordering issues.
Please test it yourself, all feedback appreciated, if you got an idea how we could implement proper z-ordering without splitting up the pillars / walls, let us know