I’ve been doing a lot of thinking around ICRPG, B/X D&D, room target numbers, and descending AC vs ascending AC, and using Player facing rolls.
First, some nomenclature so we’re on the same page…
Target = this is always 20. This is the number to meet or beat to be successful with your action.
Challenge Rating (CR) = this is how difficult a room is. The higher the number, the easier. Lower is harder. For practical purposes 10 is easiest, 0 is hardest.
Attack = D20+attack bonus+room target. If it meets or beats 20 then you successfully hit your opponent.
Defense = D20+your AC+room target. If it meets or beats 20 you take damage.
Now, in play…
Fighter (with an Attack Bonus 4 and AC 7) enters a room of CR 6 with an orc in it. The orc swings his club at him. Player rolls to defend, 11(d20)+7(AC)+6(room CR)=24. Fighter avoids the strike because he beat 20.
Fighter attacks the orc. Player rolls 6(d20)+4(AB)+6(CR)=16. Fighter misses the orc because less than 20.