Can the siege engine that powers castles and crusades be used to run 5E D&D?
I belive it can very easily. First assign prime attributes. One by class, second by choice.
Second ignore proficiency bonus (except for combat, more on this later).
If a player rolls on a skill he has add his level. Otherwise just d20+ability mod.
Proficiency bonus is now strictly to hit bonus for all classes.
Thats really it.
Now here is a twist:
Prime attributes add level to rolls.
Having a skill or not decides the difficulty of all rolls. 18 for not having a skill, 12 if you have it.