Book of War: Cavalry

Once you get past the Book of War Core Rules, then there's a "Basic Rules" section which covers historical types of normal men, terrain setup, basic formation and movement issues, and a simple morale mechanic new to the game. To some extent this is an homage to how Chainmail was set up, but more importantly it's just a really good way to introduce the game in a way that's both short and coherent (both thematically and mechanically).

Let's talk about how cavalry work. Last winter I asked some questions here about the preferred adjudications for some of the parts of D&D which are ambiguous in this respect. Here are those questions again, and the apparent consensus that I could see from the comments section (in bold):

  1. Should the modifier for mounted-vs-foot be doubled (+1 to +2) if we use it in the context of D&D? Mixed.
  2. Can any of the following ignore the rider AC bonus: (a) footmen with polearms, (b) archers, (c) giants? Yes.
  3. If a footman's attack misses because of the mounted modifier, does it hit the horse? No.
  4. Can men opt to intentionally attack the horse instead of the rider (and is there any symmetric modifier or chance to hit the rider)? Yes.
  5. Can unhorsing be accomplished with any weapon type in OD&D? Yes.
  6. Should there be some radical change to how charge attack to-hits are adjudicated (i.e., no lance exceptionalism, use horse attack level, speed indicator, re: to-hit and damage)? Yes.
  7. How many attacks per round should horses be given -- just one? Yes.
  8. Should horses continue to be barred from any attack in the charge round? No.
  9. Should there be an "overrun" capacity in which cavalry can move/attack/move (and possibly more) within a single charge round? Yes.
  10. What level of AC should barded horses be given? Mixed.
  11. Should riders be positioned at the front or rear of the horse (i.e., can they sword-attack an enemy in front of the horse)? Varies.
  12. Do we use the "rider stun" chart from Chainmail? What if the horse is dropped in a non-intentional-unseating attack? Yes.
  13. Do warhorses attack on their own if the rider is killed or unhorsed? Do they run from the line of battle, or stand motionless? Varies (suggest morale checks).
  14. Should we use a +4 to-hit bonus for charging cavalry (doubled from Chainmail's cavalry-first-turn-bonus, p. 25), and a separate +2 bonus for anyone else charging (as per AD&D DMG p. 66)? Varies.

Fortunately, I also agreed with all of those definite answers above -- it was already being baked into the game that way, so it was nice to have additional confirmation that those were reasonable rulings. Here are some more observations:

On Cavalry Attacks

First of all, it's a complete non-starter for me to give mounted horses in combat something like 3 attacks per round. The problems with that are legion: (1) it's complicated and fiddly to roll all those dice for fairly small damage amounts, (2) it really seems unrealistic that a horse can strike with all those weapons all at the same time while the rider also strikes *, (3) it actually makes the cavalry charge less damaging than sustained combat, really missing the whole point to cavalry on the battlefield, etc., etc. For reasons like these, I'm deliriously happy to stick to OD&D with its one-attack-per-round system (and reiterated for mounts in Swords & Spells p. 18), avoiding the whole Greyhawk/AD&D attacks system in this regard.

(* Growing up on a farm, I've been knocked on my ass by horses & oxen several times. But every time it was from a single hoof shooting out in a quick, solid blow. Getting kicked twice & bit all at the same time just couldn't happen.)

So what occurs in BOW is that a rider/mount unit gets 2 attacks per turn (1d6 damage each), effectively one from the rider and one from the horse. For the horse, maybe that's one kick, or an overrun/trample type attack. You also get this same "overrun" attack from the horse even in the first round/turn of combat, which simplifies things, and basically splits the difference between the more-significant charge attack (historical?) and the more-significant sustained combat (AD&D). However, in any bad terrain (woods, hill, swamp, etc.) we assume that the horse doesn't have its footing to accomplish this, and cavalry attacks are thus halved to just 1 per turn for the unit. (See Questions #6-9 above.)

On Cavalry Defense

The defense capabilities of cavalry are a little more complicated to analyze. In short, what BOW does is give every normal man/horse combination 2 hit dice. More generally, most riders on horses double their hit dice; or for mounts that are naturally aggressive -- like wolves or dragons -- you'd add the rider & mount's hit dice. Below we'll consider some issues that led to this. (For the complications in D&D regarding cavalry defense, see survey questions #1-5, 10, 12, and 13 above.)

For Question #1 (appropriate rider AC bonus; Chainmail gives +1 on p. 26), we've said previously that Chainmail bonuses should be doubled for D&D play. Indeed, a D&D +1 bonus would be too small to make any difference at the BOW d6 scale (we usually divide modifiers by 3 and round down), although I'm willing to round up from +2 in certain special cases. I ran simulations in RPGBattle of possibly using a +2, +4, or +6 rider AC bonus; one problem that came out of that is the heavy cavalry types (in plate & shield, AC2) would become totally invulnerable to normal men at anything over +2 (like, +4 bonus = AC -2), so I decided to assume a +2 AC rider bonus.

But what does that indicate, really? For standard melee types, it's a +2 AC bonus for reach/cover from the horse itself, while trying to strike the rider. (Note, however, that we do not assume resulting misses effectively strike the horse -- Question #3 came out "no" -- and similarly, the original Chainmail rule doesn't have any comment or method for handling hits on horses.) However, for missile attacks falling from above, we don't assume that cover applies, but instead that we're rolling a randomized 50/50% chance to target either the rider or the horse (similar to missile discharge in DMG p. 63). Fortunately, either mechanic is approximately the same: Say, for the heavy cavalryman (AC2), normal men have 4 pips to hit on d20 (to-hit 17+), so reducing that by 2 points is the same as a 50% hit reduction. For other armors it's skewed differently, but still approximately correct, so we're comfortable assuming this for either type of attack.

One alternative we considered is to use the +2 AC bonus directly (rounded up, so +1 AH in BOW), but the problem with that is that it makes heavy cavalry AH 7 (i.e., D&D to-hit 19, about one-half chance in d6). If we did that, then we'd need a mechanic that says something like "AH 7 means a hit roll of 6, followed by a 2nd confirm roll of 4-6", which is something they actually do in Warhammer. But (a) my playtesters didn't like that, (b) the extra-die rolls seem complicated, clunky, and inelegant, and (c) the mechanic doesn't scale smoothly to ultra-high ACs you'll have to deal with for high-level heroes. So that was rejected in favor of the 50/50 hit assumption, i.e., cavalry act as though they have 2 Hit Dice, which is statistically about the same anyway.

On the other side of the equation, you have to ask: What's the best strategy for people attacking cavalry; should they target attacks first at the rider, or the horse? One assumption we make is that if the rider goes down, then the horse either stands nearby quietly or runs off (i.e., doesn't actually press the assault while riderless). So the question is really what eliminates the rider faster; perhaps killing the horse first and removing the +2 AC bonus (or whatever) is the better strategy? So that's a strategy question I ran though RPGBattle -- and as might be expected with a 2HD or 3HD horse in D&D, it turns out not to be the case. See the spreadsheet below for results of that analysis; even if we engage both the Chainmail rider-throw table (stun thrown riders for 0-3 rounds; p. 26) and AD&D stunned-to-hit bonus (+4), it's still always better to attack the rider directly than to kill the horse first. ** (And therefore, those latter mechanics [available by switches in the RPGBattle program], and any option to attack the horse [Question #4 above], turn out to be of academic interest only.) The only time when that wouldn't be the case is if we dialed up the rider AC bonus so high (+4 or +6) that the rider is actually immune to attacks entirely. Fortunately, that's also in synch with Chainmail, which as we said before has no comment on any option other than attacking the rider in a cavalry-melee situation (p. 26), bolstering our model here as true to the core D&D system.

(** Now, my dad, who's a large-animal veterinarian, will watch a Western chase scene with me and claim that "it would be much easier to stop that guy by shooting at the horse's leg than the rider", but I'm not so sure about that, since it seems like a really small target. But I wanted to consider it for D&D nonetheless.)

So that's where our BOW model of cavalry hit dice comes from; per D&D, it's really about twice as hard to hit the rider, and therefore effectively takes twice as many good hits to eliminate the unit. Two other details: (1) Barding, following Chainmail, will be assumed to be of only one type that gives +2 AC to the horse, in use for heavy cavalry (see Question #10); therefore horses are all within 2 points of the rider AC, and we say that's "approximately equal" for our purposes at mass scale (and even if doubled to +4, it would be an irrelevant change anyway). (2) Pikes will be given a D&D +2 to hit cavalry (+1 in BOW) to reflect the fact that their long weapons negate the reach/cover bonus that riders normally get from melee-types (see Question #2).

Compare what we've done here to other D&D-branded wargames: Gygax's Swords & Spells directs you to add up all of the hit points of riders & mounts [p. 17], and to defeat them, you've got to degrade all of those summed hit points; personally, I think that's a grave mistake, as you clearly don't need to murder every single limb and cell of a cavalry unit in order to defeat it (again, if you knock off all the riders, the horses -- the majority of the summed hit points, after all -- aren't going to keep leading the attack against you). From my youthful play experience, cavalry in Swords & Spells took enormous, crazy amounts of damage before you could defeat them. In contrast, Doug Niles' Battlesystem instead gives a rule of "average the rider's HD and the mount's HD; round up" [1st Edition p. 19; 2nd Edition p. 105]. That's a distinct improvement, but I think it still runs into problems if you have a very large, non-aggressive mount (like an elephant, perhaps) and a normal man riding it; really, all you have to do is kill the 1 HD man to defeat the unit, and it seems like an overinflated bonus to give him half the mount's HD as a modifier. So that's why I prefer the rule in my Book of War: double the rider's HD for non-aggressive mounts (bonus capped by the horse's HD; reflecting a +2AC or 50% targeting miss chance), or sum the rider & mount HD only in cases of naturally aggressive, monstrous-type mounts. I'm hoping that you'll agree.


  1. CHAINMAIL does talk about killing horse and rider (seen more as an abstract of simple casualty, only 1/3 of casualties were actual deaths according to the rules).

    If you look at the longbowman write up in chainmail, stakes planted in the ground will kill the horse and throw the rider. CHAINMAIL mass combat doesn't concern itself with figuring out if the rider is dead, it simpl treats him as a casualty and no longer in battle.

  2. I'm talking man-to-man rules here [p. 26], i.e., the infrastructure of D&D combat.

  3. I've been enjoying your posts where you explore the system.. Just ordered up a BoW.

    The combined HD compromise seems to work. Not too hard to remember, and it's a nice chain of reasoning to get there.

  4. K. Bailey: Thanks for the comment! Makes me glad, because there's more to come. :-)

  5. I haven't yet had a chance to purchase BoW (though I will… oh, yes I will), but this discussion leaves me with a question: what about an instance of high HD rider and aggressive mount? For instance, say an 8th level Fighting Man is riding a 5HD Lion (fantasy worlds can have some strange and wonderful combinations). Would the total HD for the cavalry figure be 16 or only 13? In other words, would the high level Fighting Man would be better off riding a 3HD Horse than a 5HD aggressive Lion?

  6. faoladh -- Good question. Check out the detail in the second-to-last line of the blog post (and this is in the book on p. 10). Superhero on a horse would be represented by HD11 (the bonus is capped by the horse's HD); superhero on a lion would be HD13 (and what a great image that is!).

    Now, the fine print on this is that to actually see either HD11 or HD13 at BOW scale you'd have to be talking about a full squad of 10 of these superheroes-on-mounts. If it's actually just one individual, you have to divide by 10 and round down, so in either case he'd appear as just HD1 at BOW scale. (That is, 1 HD at BOW scale is always 10 HD minimum at D&D scale).

  7. Excellent! Thank you, and I apologize for missing that last reference in the blog entry.

  8. Yeah, and that was really on my end, where I tried to extra-abbreviate the rule at the top of the blog post. Even in the book itself I've gone around a couple times trying to find the shortest, clearest expression of the rule. Like, you could also say: (1) "Add rider & mount HD (but for non-aggressive mounts, the latter term shall not exceed rider HD)", or (2) "Take rider HD and add a bonus equal to the lesser of mount or rider HD (but for naturally aggressive mounts simply add rider to mount HD)".