After reading through the Mutants and masterminds 3E players book, I was reminded how this game played using a single d20. Which is not new to me. I ran M&M 2E and True20 before that.
So I got thinking. Why couldn't I do the same with Castles and Crusades or Amazing Adventures? After all the core of the Siege system is a single d20 roll. The main use any other die roll is variable damage.
So I started thinking. if I was to use just a d20 to run C&C and AA how would I do it?
I would make all damage equal to the average of a the die roll. 2 for d4, 3 for d6, 4 for d8, 5 for d10, and 6 for d12. And bonus would still be added. So d6+2 would be 5.
On any roll to hit that is 5 greater than the roll needed the damage would instead be maximum damage. 4 on a d4 or 6 on a d6...ect ect.
On a natural 20 damage will be 1.5 of max damage. 6 on a d4, 9 on a d6, 12 on a d8..ect, ect.