For catwalks, you should use both:

- building:min_level
   - The "floor" of the catwalk starts above X floor...
- building:levels
   - The "ceiling" of the catwalk stops on X floor...

For more details, see:

- https://wiki.openstreetmap.org/wiki/Key:building:min_level

- - -

In this case, it looks like it is:

- Just 1 floor tall.
- Attaches 2nd floor of each building.

So it would probably be a skinny rectangle that has:

- building:min_level=1
- building:levels=2

which says:

- "The floor of the catwalk starts above 1st floor."
- "The ceiling of the catwalk stops at 2nd floor."