Running 5E D&D using the siege engine.

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.

Faceless Ones (Basic fantasy)

Faceless Ones

AC: 12
HD: 1
# of Att 2
Damage: claw/claw d6/d6
Move:20' walk / 40'leap 
# appearing: 
Save: as 5th level Cleric
Morale:12
Treasure: None

XP: 25 each

Faceless ones are summoned by dark sorcerers as minions. Even those that summon them do not know from what foul realm they call home. The spell to summon them is a two part spell. The first is to summon, the second is to control and bind. Once summoned and bound the faceless ones will follow the command of the one who bound them. Faceless ones can track any target by smell. They tend to walk with a very slow and shambling gate. Once they have sight of their prey or target they sprint and leap in close to attack.

GM: This is not intended to be a spell for the players to learn and use. It's dark magics used by only those with villainous intentions. If you do intend to allow players this spell I would recommend it as a scroll or consumable use spell.

Summoning Faceless ones: The spell to summon is a quick one. A summoning circle is made and the incantation is spoken. At this time the number of faceless ones is chosen. T caster must immediately give their own blood and life force to control the faceless ones. Their blood is dripped on the summoning circle. The caster gives one hit point per faceless one chooses to control. Only then can the 15min incantation to control be said. If for any reason the caster is interrupted the Faceless ones break free and will attack anyone in sight including the summoner.

Adrenaline Rush, Fighter (C&C)

New ability for fighters.

At level 3 fighters gain the ability to call on their reserve of fortitude to see themselves through. Once per battle at the start of their turn when they start with half their hit points or less they can activate adrenalin rush.

Adrenaline rush allows the recovery of 1HD+Con mod. In hit points. As well as +2 damage to the first attack that same turn.