Good fun puzzle thanks Nicola.
A quick hint in case anyone makes my mistake.
Let’s say the possible floor levels are Low, Floor, High.
For bridge (X) with description “you can walk under and over”, this means ‘Floor’ and ‘High’.
I mistakenly thought it meant ‘Low’ and ‘High’. 
But it’s possible floor levels Low, Floor, and High? I use only floors: Zero (Floor, Tunnel, Bridge, if before bridge cell is level Zero) and One (Short_wall, Bridge, if before bridge cell is level One). And slopes change level from Zero to One and from One to Zero. But validator’s fourth testcase is not solved. If I think that Tunnel is level Minus_One (you say Low), should I use slopes to come in and come out tunnel?
I don’t think that this case happens.
I’m also blocked on fourth test. Not the one of test cases but the one for submission (So I can’t check where is the problem)
I’ve a very clear code that answer to all constraints. So the problem seems to be a misunderstanding of these constraints. But as it is clear for me, I’ve no track to check. I these conditions, I give up. 
Can you send me your code in private?
I’ll send back what’s wrong.
(Note: not meant as criticism, just a comment/suggestion 
)
I enjoyed this problem, but thought it’d be nice if problems that are going to be “featured puzzles” (and possibly seen by a lot of people), would be checked and cleaned up a bit (if necessary) before they go “live” as featured.
Came to mind because on this one people seem to be getting stuck with ‘Validation 4’ (I failed that one too at first), and the problem currently states, “Note The puzzle is currently edited because: …” (made me wonder if I should wait for it to be edited). Other parts of the puzzle’s description could be a bit clearer, too.