It can all be approximated by comparison to Regulus' range. I don't play Demigod anymore (just browse the forums while reading), so I can't test for the exact ranges, but...
Light towers have either regulus' range or regulus' range +1. This is apparent because regulus cannot outrange the tower without his first range modifier (which is a +2, as are all his range modifiers, iirc), but can do so with it.
Archer towers have either regulus' range +2 or +3, following the same logic.
Fortresses have +4 or +5.
In order to test the -exact- range, you could have a level 1/3/5 (or whatever the progression is) regulus walk up to one of the towers and autoattack when his creeps aren't around. Does he get hit first, or does he attack at the same time as the tower? If he gets hit first, the towers have +1/+3/+5 range over the standard reg. If he attacks at the same time, +0/+2/+4. Hope this helps.