Just read this the section on large balkas:
Tanks and guns may fire out of a -1L balka hex at these
bordering hexes, but not into a balka hex from a bordering hex. All
other units may fire into balkas from bordering hexes and vice versa.
Designer’s Note: Tanks could raise their main guns quite high,
but could not depress them enough to fire into a deep balka.
I would have expected this to also apply to tanks on hills firing into lower elevations, but that doesn't seem to be the case. I suppose balkas are treated differently because their walls are much steeper than your average hill.
Also, am I correct in my understanding that this firing restriction for tanks/guns is applicable to all
large balka hexes (as opposed to just an immediately adjacent balka hex)? And are they applicable to a tank sitting next to a balka opening
Apologies if this has been covered before.