Accelerated D&D Combat, Platemail-Style

So, a couple of days ago, I discovered Platemail 27th Edition. It's, loosely speaking, a retro-clone of Chainmail, though (as far as I understand -- I've never actually read Chainmail) it's greatly expanded, as Chainmail wasn't really a full RPG.

This link seems to contain the most up-to-date versions of the rules: http://furiouslyeclectic.com/forum/printthread.php?tid=190

Direct links to the main rule book: https://drive.google.com/file/d/0BxVXsD3On015OTNTclgySmFCamc

and the book of magic: https://drive.google.com/file/d/0BxVXsD3On015b09Nd1hmX18wX3M

It's got some really nice touches and I'd definitely recommend you to check it out. Some highlights for me include:

• The book of magic. There are some super nice, flavourful spells in here. Not just rehashes of the classic D&D spells (though, naturally, some of them do make an appearance).
• The rules for divine magic. There's no cleric class, but any character who takes the Devout ability is able to petition their god for magical aid. It's a really nice system.
• The skirmish combat rules. These are what initially brought me to wanting to look into Chainmail, in the first place, as I will elaborate below.

Skirmish Combat for D&D

One thing that I find irksome about D&D, from time to time, is the amount of time that resolving combat can end up taking in sessions. Even in simple, old-school systems, a fight can easily take half an hour (or longer). Sometimes this is fine, but at other times -- especially when several combats happen in a single session -- I find myself wanting something quicker. The original idea I had was the possibility of a system where a complete battle can be resolved in a single step, without requiring individual rounds/actions to be resolved. I've not managed to come up with anything so greatly simplified (yet), but I did come up with a system for D&D combat, inspired by Platemail's skirmish system, that I reckon should be significantly quicker than the standard D&D combat system.

Here goes:

Hit Dice
In addition to his or her hit points, each character has a number of hit dice. These depend on class and level (fighters have one HD per level, rogues have about one per two levels, wizards have about one every three levels -- proper progression charts needed). Hit dice represent both a character's ability to avoid dying and to inflict death upon others.

Action in Combat
Combat rounds are of indefinite length, somewhere between ten seconds and a minute. Many attacks may be made each round. Each round of combat:

• Each character can move, attack, or cast a spell.
• Move: up to full movement rate.
• Attack: make a number of attack rolls equal to current hit dice. Apply all normal modifiers, but THAC0 is always 19 (the same as a first level character). Attacks are directed at groups of enemies with the same AC. If different groups are present, attacks may be divided between them. Each attack roll that succeeds is one hit. See damage, below.
• Casting spells: healing spells restore one hit die per die of healing. Damaging spells inflict one hit per die of damage.

Damage and Death
Each hit reduces the enemy by one hit die -- either defeating an individual with one hit die or reducing a more potent foe's total by one.

Defeated characters may not move or act in any way. Defeated monsters are assumed to be dead or shortly on the way to death. Defeated PCs have a chance of survival, see below.

At the end of combat, if enemies remain, defeated characters may be:

• Killed: eaten, finished off, dismembered, sacrificed.
• Captured.
• Ignored.

A PC who is lucky enough to be ignored or forgotten after being defeated in battle may make a saving throw versus death to determine his fate. If the save fails, the character is dead. If it succeeds, he is alive with 1hp and an injury which permanently reduces a random ability score by one. (A "death & dismemberment" table may be used at this stage, instead.)

After a battle, undefeated PCs lose 1d6 hit points per hit die lost during the fight. If this brings the character's total to zero or lower, a saving throw may be made, as above.

Recovering Hit Dice
Hit dice can be lost during combat and are recovered after a night's rest.

Analysis
I've yet to try this system, so can only speak theoretically, at this stage. I feel it should be quicker in the following ways:

• No initiative roll. A minor thing, but every "roll this per round" makes a difference.
• No damage rolls. This is the big one. Hit points are abstracted into hit dice, making damage rolls redundant. Every hit simply reduces the target by one hit die. The conversion from HD to hp is done at the end of the battle -- either via the death save (-> 0 or 1hp) for defeated character or applied as hit point damage to undefeated characters.
• Resolving multiple attacks per round. Characters with more than one HD can make multiple attacks per round. This should greatly speed up resolution, as lesser enemies may be mown down. This of course changes the balance of combat and may skew it too heavily in favour of those with higher HD. I also don't think this scales up to the standard D&D level range -- a 15th level fighter rolling 15d20 per round?? That probably wouldn't work so well.