Simply put, ACKS is like the old Basic and Expert D&D (generally called B/X D&D) by Moldvay and Cook with some stuff from other editions, including 3rd, grafted on. It does some things of its own, like using a different to hit method in combat, and has a focus, at least in theory, on ruling domains (either a kingdom or a criminal or commercial enterprise), though it's not all that much deeper (or different) than the rules found in the Companion and other D&D sets.
Since ACKS is based on B/X (or rather, Labyrinth Lord, a clone of B/X), you have race integrated into class. Besides the very basics of fighter, magic-user, cleric, and thief, the classes are not the usual B/X ones. You have two OD&D/AD&D classes, the Bard and Assassin, and some brand new ones like the Bladedancer (sort of a dervish, a cleric that uses swords and leather armor) and the Explorer (kind of like a thief but not so larcenous).
But beyond that, the racial classes give some choice other than just Elf or Dwarf. Elves have the Spellsword and Nightblade to choose from, basically a Fighter/Magic-User or Thief/Magic-User. Spellswords seem to get the better deal for the Elf, they fight better and cast much better, yet their advancement costs less experience. Depends on how much you value thief skills, I guess. Dwarves get the Craftpriest, basically a cleric, and Vaultguard, basically a fighter.
Want to play something else besides a human, dwarf, or elf? Well, you're out of luck unless you buy the Player's Companion, which has a class for gnome and lizardman (plus 3 more each for the Elf and Dwarf, and a bunch more for humans). I'll probably review that in the future, but it's pretty essential for an ACKS game, as it adds a lot of stuff that the core book is missing.
Attack and spell progression more or less follow B/X D&D standards. As mentioned, determining whether or not you hit is, as far as I know, unique to ACKS. It's a variant of THAC0, which was a shorthand way of not having to use combat tables in AD&D/D&D. To Hit Armor Class 0 it stood for, and it was a number you needed to roll to hit that Armor Class (AC), and for other ACs, you simply subtracted the AC from THAC0. For instance, most 1st level characters have a THAC0 of 20, so to hit an unamored opponent of AC10 (the worst armor class), you would simply subtract 10 from the THACO of 20 and need to roll a 10.
In ACKS though, they've simplified things somewhat by having armor class start at 0, with it being the worst, not 10. So a starting character has an Attack Throw of 10. As Armor Class is then simply added to the number needed, so in this case, you add the zero to the number needed, which gives the same result, ten, only by adding instead of subtracting. The Attack Throw decreases as characters gain levels.
As an example, if you have a 4th level Fighter with an Attack Throw of 8, you would need to roll a 12 in order to hit an Ogre, which has an AC of 4 (8+4). By comparison, in ascending armor class, an Ogre has an AC of 14, which is the total needed to hit him, a player rolling a d20 and simply adding his attack bonus of +2. While the math is the same, it seems like ACKS ways means the DM does the work (unless he tells the players the AC), while the ascending way the player does the work, not needing to know the AC.
While the ACKS method certainly works, on the flip side it has the effect of constraining the game in terms of levels. Since otherwise the Attack Roll would reach 0 or negative numbers. In AD&D, for instance, a Fighter's THACO starts at 20 and can go as low as 3, which in ACKS terms would be -7, and in Rules Cyclopedia D&D, it can hit 2, or -8 in ACKS. But ACKS avoids this problem by stopping level advancement at 14th level.
The level cap also restricts the spells in the game to 6th level, but some of the more powerful spells can still be learned and cast as rituals at 11th level. While this gives 11th level character access to very high level magic, at the same time, it's a very limited number, they take weeks to cast, and cost a lot of money.
Spellcasters (mostly magic-users) also can create magic items and things like constucts, with other classes mostly being out of luck.
Alignment can be a thorny issues in D&D style games. Being largely a B/X clone, ACKS uses the single axis alignment system, of Chaos, Law, and Neutrality. But where the original alignment system offered quite a deal of ambiguity, much like how it did in Michael Moorcock's writings (where it's largely derived from), ACKS equates Law to Good, Chaos to Evil, and Neutrality to Selfishness.
Skills & Customization
The main way of customizing a character in ACKS are proficiencies, which in D&D terms, are like a combination of feats (from 3rd edition), class powers, and skills. Each class has a different list of ones they may take, and there is a general list that all classes can pick from. Characters gain one from each at 1st level, then a class one every 3-4 levels, and a general every 4 levels.
Skills, either gained through proficiencies or from a class (i.e. thief) are similar to the attack rolls, having to roll a d20 and beating a target number, which is determined by either the level (for class skills) or fixed, in the skill description.
For example, a 1st level thief needs to roll an 18 or better on a d20 to open a lock. At 15%, that's a bit lower than a 1st level AD&D thief's 25%, but equal to the D&D thief, either OD&D or Basic. Still, hope you didn't want too many locks opened.
Non-class skills learned as proficiencies are largely self contained, following their own special rules. For instance, Healing can be taken three times, and can produce various effects, such as healing damage, neutralizing poison, or cure disease.
In a lot of ways, this was supposed to be the centerpiece of the game, but the rules are a bit of a letdown in practice. Not much of an improvement over the old Companion rules in terms of page count, and I'm not sure much actual improvement. Indeed, they bear a great similarity to the rules in the Companion set (or Rule Cyclopedia), right down to the contruction costs table, and the basis of domain income being based on how many peasant families reside in it.
Basically, a character controls a 6 mile hex on an outdoor map, and gets 3d3 gold pieces per month (randomly determined when the domain is founded) per peasant family, with the number of peasants depending on how civilized the place is (10-30 in the wilderness, 30-180 in borderlands, and 80 to 480 in civilized regions).
The population just fluctuates randomly if left on its own, but the owner can spend 1000 gp a month to make agricultural improvements that will attract more people. Somewhat counter-intuitively, more people will also arrive if the player adventures.
The other thing that can be done is founding an urban settlement, but this basically is just another domain within the domain. Again, the player just gets a random number of people, then increases this by spending 1000 gp a month.
Quite honestly, I was disappointed with the rules. I think it would be better to adapt the rules from Pathfinder's Ultimate Campaign (or Kingmaker), if you are just looking for rules for domain management.
It does go into details for non-kingdoms pretty well, like if you want to run a thieves guild, or run a mercantile adventure where you trade from city to city. But I believe one of the D&D Gazetteers had rules for that. In any event, it seems more like a spreadsheet than a game.
Monsters and Magic Items
Essentially a rehash of the usual D&D monsters, converted to ACKS form. It's not a particularly comprehensive list, maybe about 150 monsters. By contrast the original AD&D MM had 350, and Blood & Treasure has roughly 500 (basically everything in the SRD, plus replacements for classic monsters not in the SRD, plus some original games).
And then a selection of the usual D&D magic items, a fairly abreviated list. Some of the most interesting magic items are missing, too.
The layout is clean and very readable. I think they must have spent most the art budget on the cover. The interior art isn't bad, but a huge drop off in quality from the very snazzy cover that makes it look worse in comparison.
Adventurer Conqueror King System has some lofty ambitions, but largely fails to deliver on them. Partly this is because it was too ambitious to simply fit into one book. True, the Rules Cyclopedia covers most the same ground (and more, like actually having mass combat rules) in one tome, but that had three columns of text as opposed to two.
Rather than try to cram everything into one slim book and skimping on a lot of stuff, they would have been better off either following the classic pattern of Player's Handbook, DM's Guide, and Monster Manual, or by putting out 3 books, each based one word (Adventurer, Conqueror, King), not unlike the BECMI boxed sets.
Especially as they seem to be putting out 3 books anyway, the first book being a player's companion (adding more classes and spells, basically essential for play, IMHO) and the second on warfare. (I imagine there would also eventually have to be a monster book, given the dearth of them.)
And since they decided to go with their own method of to hit determination, you have to convert from pretty much all existing D&D material, both new and old school. Okay, it's not that hard to convert, but it's still extra work.
So out of the three words in its name, it does one okay (Adventurer), one not so okay (King), and one not at all (Conqueror). It's worth a look if you want an old school D&D game with some customization (Blood & Treasure is also good for this), but it seems less than complete (unlike the aforementioned Blood & Treasure). C+