Flavor
Vikings, Saxons and Samurai battle over the ruins of Greek and Persia.

Time period
The year is 1000. Wikipedia is my main source book.

Geography
Japan, Scandinavia, Britiain, the Asian steppes and the Byzantine Empire are projected onto the Philippines (turned 90 degrees counter-clockwise). I don't use a hex-map but measure distances directly in Google Earth.

Alignments
Law - seeks transcendence and to remake society according to divine principles. Buddhism and Christianity are the major forces of Law.
Neutrality - seeks glory in the mortal realm. Shintoism and Norse heathenry are the major forces of Neutrality.
Chaos - seeks the physical and spiritual fall of man. The Nephilim occypying the former Byzantine Empire are the major force of Chaos.

Language
A major theme in the campaign. Most people and many monsters encountered speak Common (Sabir) but adressing someone in their mother tongue (or the language of their arcane order, or thieves' cant in the case of mages and thieves) provides a bonus to the reaction roll. Learning a language takes 20-Intelligence weeks of interacting with the corresponding culture and you can know 6 modified by INT languages at one time.

Big Things
In Constantinople the Nephilim are building a colossal tower, each year it gets closer to heaven.

The enchanted peoples' displaced by humanity thousands of years ago are stirring. The ancient pacts and seals forged between them and the pagan religions are either neglected or not maintained when the lawful religions dictate their abandonment.

Rule set
Adventurer Conqueror King with a few twists.

When creating a character you roll 5 sets of stats and pick one as in ACKS (the rest are assigned to starting henchmen if any) but you're not allowed to modify any stats. Characters start with maximum hit points and all HD are rerolled each level (a low roll means they gain 1 hit point). Instead of picking proficiencies like in ACKS you pick a kit you can afford with your starting gold, each kit comes with a minor special ability and some gear. The selection of kits expand as the players encounter new groups and manage to ally with them (to play a non-human they would have to find and ally with non-humans first). Two examples of special abilites:

Ronin are Faithless, this allows them to backstab as a thief of the same level and to change their alignment once per day.

Hedge Knights can take on a 0th level human as a Squire, this does not count towards their henchman limit. Squires have +4 to loyalty rolls and require no salary or treasure share, only 3gp/month in upkeep. After 1st level the player may bestow knighthood on the squire turning him into a normal henchman, but he retains the bonus to loyalty (morale, loyalty and reactions rolls are done with 3D6 and a wider spread so a +4 bonus is more like a +2,7 bonus using the standard tables).

Healing
After a fight, if 1 turn and 1 pint of water is spent resting characters regain 1d3 hit points (modified by CON) lost during the fight.

Clerics
Can't turn undead. Instead they can Invoke the Divine once per day to gain 1d6+1 temporary hit points, immunity to fear and delaying poison for 1 turn. At 4th level they may Invoke the Divine twice daily and 10th level trice daily. The hit point boost is increased to 2d6+1 at 7th level and 3d6+1 at 13th level. The better of DEX and WIS grants a bonus to initiative, thus the role of the cleric is as vanguard, charging in first knowing that God will protect them.

Thieves
Their skills count as if they were 4 levels higher.

Mages
Can cast cantrip versions (called bodings) of their known spells if they have not been cast that day. A sleep boding affects 1 creature of 1HD or less up to 5' away, a shield boding lasts as long as the mage concentrates and a detect magic boding requires touching the studied object for a turn.

Spell variants
The standard spells come is several variants, favored by different schools. The Black Sleep affects only 2d4 HD but the duration is a full day and only wounds can wake the victims. The Secret of the Radiant Eye is a magic missile stored in an eyeball. It's range is only 60' but as it is cast during the storage process the bearer can unleash it as a missile attack.

Weapons & Armor
Shields grant +2 AC if wearing mail or lighter body armor.

Classes are not restricted in their weapon or armor use. Mages get a penalty to their initative when casting for each point of armor they are wearing, staffs provide a bonus to initiative when casting (for clerics too). Thieves get a penalty to their skills if wearing armor heavier than leather.

Weapons have a long list of minor bonuses and drawbacks, I can post it if someone is interested. Plate armor doesn't exist but full body mail functions the same.

The campaign so far
12 sessions in, the players have dipped their toes in various dungeons. They have lost 5 PCs and 2 henchmen. To sell a magic item they had to travel to the largest city in the region, Heian-kyo and have been doing urban adventures in the form of burglarizing mages and sucking up to the nobility. Three PCs hit 2nd level recently. Heian-kyo was built according to geomantic principles, the city acts as a seal keeping the horrors of Stonehell locked up, but as large parts of the city are derelict the magic of the seal is fading. I'm sure the players will be drawn to it when the notice the meager rewards of intrigue, the minor nobles who will pay them attention only have hundreds of gp in monthly income, a few hours of dungeon crawling can net thousands of gp.

I have the biggest boner for your game. Also, you reminded me that I need to buy Stonehell...