What determines whether a unit can fortify?

[Telecide]

Mech infantry can. Modern armor cannot. But I haven't been able to find where this setting is.

 

[mechaerik]

Whether or not they receive defensive bonuses.

 

[Telecide]

Whether or not they receive defensive bonuses.

Thank you!!! I've been trying to figure this out for months! I always thought it had to do with unitcombat. So that's the bNoDefensiveBonus in Civ4UnitInfos.xml?