In particular: Among my design goals for BOW were that it should be short, concise, stand as a coherent game unto itself, and still be manageable by complete newcomers -- high-level D&D wizards seeming to contradict all of these points.
But granted that (working at 1:10 scale) individual heroes really have to be over level 10 in order to appear on the board as specialized figures, it dawned on me that this synchronizes with the original D&D rules in some very nice ways. First: If we require that any heroic magic-users be level 11+ before we deal with them, then this is exactly "name level" in OD&D, and calling them and treating them as "wizards" is in fact accurate. Second: If we restrict ourselves only to the top-most level of spells (6th) -- the ones most likely to be used in extremis -- then we see that OD&D wizards gain exactly 1 of these spells per level above our basis (0 at 11th, 1 at 12th, 2 at 13th, etc.*).
Therefore, restricting BOW wizard powers to only 6th-level spells has the following advantages: (a) It reduces the spells that we need to present to a single, easily-digested page. (b) It throws a spotlight specifically on those powers which include most of the great battlefield-changing effects (lower water, move earth, control weather, etc.). (c) I'm able to entirely avoid referencing the D&D experience level tables, and instead simply refer to wizards of "rank" 1 or more (really levels above 10), with an associated number of "greater spells" (6th level spells: of number one less than rank).
This started to make me very happy. Now, 6th-level spells are not purely what we limit BOW wizards to: We also give them certain low-level protections by assumption (shield, protection from evil, detect invisible, and some sort of magical escape ability). And furthermore: Assume that any of them bring some sort of wand of fireballs or lightning bolts to the battle -- such that we can generally assume their ability to sling one of these 3rd-level spells at-will for the length of the battle. In other words: Their profile looks very much like it did in the original Chainmail Fantasy.
So, here's the core of the rules for Book of War wizards, with their three main categories of powers. (Text between the rules below is indicated as Open Game Content per the OGL):
Basic Abilities: We assume that any Wizard has a number of low-level spells active or available on the battlefield. Each has a magic shield (AH 6+), a protection from evil ward (cannot be hit by enchanted monsters, etc.), and can detect invisible (see any hidden or invisible creatures within 12"). All have at least one of invisibility, flying, or polymorph, so they can escape from the battlefield whenever they wish.
Magic Wands: Wizards are further assumed to have either a wand of fireballs or lightning bolts. This gives the Wizard a magic area attack that affects one figure, with range 24", damage 6, and firing 2/turn (or 1 with up to a half-move; blocked by woods and hills). Roll one die for accuracy: 1-2, 1" short; 3-4, on target; 5-6, 1" long. Assume that wands have sufficient charges to last the length of a normal game.
Greater Spells: Wizards above the 1st rank can prepare a number of "greater spells" (6th-level) equal to one less than their rank number; e.g., a 4th-rank Wizard has 3 greater spells. The Wizard can cast one of these spells per turn, standing motionless to do so. All effects commence on the Wizard's attack phase. Choose from this list...
The list that follows includes the 6 topmost spells from OD&D that I felt most applicable to the battlefield (affecting large terrain or many men, etc.), each with a tactical effect described in three lines. The fireball effect shown above was developed after lengthy consideration here.
As you might expect, this is one section in Book of War that concludes with an open-ended option to the effect that "you're free to use the full arsenal of D&D spells in your game", so long as you take the initiative in converting all those other (non-fireball, non-6th level) spells, and also to increase the cost-value of your wizards appropriately (although that can be a non-issue if you're playing a campaign game, instead of standalone points-based battle). Perhaps this might be an area where we could consider a further expansion in the future: a book that converts all OD&D wizard spells to Book of War statistics.
However, in practice we've found that this model (a) provides as much as a wizard gets or wants to use during a standard battle, (b) is tractable to a balancing analysis the generates a useful cost-value, and (c) is comprehensible to new players who are using or confronting wizards for the first time (those completely unfamiliar with high-level D&D play, or even players predisposed against complicated magic rules based on prior experience). Hopefully, you will, too.
* Fine print in OD&D text indicates that after 16th level, wizards gain an extra 6th-level spell every 2 levels [OD&D, Vol-1, p. 19].
[Photo by fuzzcat, under CC2.]