I was wondering, is there a way to do a defense system in an rpg without actually having to upgrade your defense or creating more units? If so, can you please help me and explain them to me.
Well you could use virtual HP combined with randomization of hit %. As you get more defense, make the % of hitting your unit lower, to represent more "dodging" skill.
Have a bunch of triggers that level up the unit's max health every once in a while. Like if switch one is set and the unit steps on "heal", set it's HP% to 50. If switch 1 is cleared and switch 2 is set, set the HP % to 60.
This won't work if you've got medics on the map, though.