How many z-levels can a marksdwarf engage from?

If I was to build a turret defence along my fortress perimeter, how high could I make it before my stationed dwarves armed with ranged weapons couldn't engage a target on the ground?

Corollary: does the range at which they can shoot from, in both x/y and z-levels, increase with their skill in archery, marksdwarfship and bowmanship?

The range of dwarfs does increase with skill, which means that the max tower height is dependent on your dwarfs skills. I feel that the range varies from around 20-50 horizontally. Can't really cite this, just going on my experience. Maximum range would be roughly 50 tiles vertically and 50 horizontally. I seem to remember reading that range is based on horizontal distance. That is, shooting 50 tiles forward is no different from shooting 50 forward and 50 vertically.

If you have Legendary+5 marksdwarves, a 40+tower might work... but only hit things 40+ tiles from your tower. The most efficient fortification is only 1-2 Z-levels tall.