I believe the tortle code is having issues right now due to some recent patches from LW and might not be the best example.
The tortle special ability Natural Armor has an eval script with the line:
Code:
hero.child[ArmorClass].field[tACNatural].value = 7
You have to change the 7 to a 4 in your case.
However, there is a new way to do it. If you create a totally new (blank) special ability and call it Natural Armor you can find the section called "Armor Class Calculation" down toward the bottom of the editor.
You could put Base Value 14 and hit the button for AC Modifiers and check Constitution Modifier and Shield (which would allow only CON and shield) to calculate this AC value.
I think you'll still have a problem with Armor and with Monk/Barbarian abilities though. HL chooses the best AC from all the AC calculations, so if your natural armor ac is 16 and your character puts on plate (ac 17) it grants the higher ac. We tried to address this in the eval script on the tortle Natural Armor if you want some code examples of how to negate armor bonuses on armor worn.