This is a somewhat roundabout post, but ends with putting the Conqueror back in ACKS. Bear with me.
I've been playing a lot of Europa Universalis IV recently. I'm fairly rubbish at it, but I enjoy it nonetheless (and more than its sibling from Paradox Studios, Crusader Kings II, which I've mentioned previously and am even worse at). These are both essentially domain games; Crusader Kings in particular has commonalities of scale, feudalism, and frequent assassination attempts with ACKS.
One thing I have noticed is that upgrading your lands in EU4 is a slow, expensive process, and even more so in CK2. I think the only time I had the cash on hand in Crusader Kings to upgrade one of my castles was as a Swedish viking-king who made a habit of multi-year raiding expeditions against the English and Irish coasts, and in EU4 most of the money I've spent on upgrading has come from tribute extracted from nations defeated in war. Saving cash from the normal budget for a new castle is liable in CK to be a multi-generational affair with years of build time once you've finished gathering the money, while in EU it might take only a decade of prosperous peace and a year of buildtime.
As this suggests, the big money is in warfare. Even better than temporary extraction of tribute is permanent (or hopefully-permanent) conquest of provinces and vassalization of smaller states. To use Civilization terminology, playing Tall (a few highly-developed provinces) is much inferior to playing Wide (many poorly-developed provinces).
In Civilization, most of the map is basically empty at the beginning of the game, and at least in the Civ5 games I've played buffer zones between countries are fairly common even into the late game because playing Tall works well - you settle a handful of cities in good positions and then you tech up. In the Paradox games, almost all the land is somebody's land, even if they're some backwards tribe of Siberian nomads - if you want to expand (and you do, because building up is extremely expensive), you must conquer. Conquest notably does not typically entail the extermination of the inhabitants of the land; merely their subjugation, taxation, and drafting for future wars. In that ten years of time it would've taken you to fund and build a new castle, you could instead take two of your neighbor's castles, and those come with land and serfs and tribute! Sure, they might rebel later, but that's what you've got a garrison for, right?
To get to the point, finally - when I ran the early domain game in ACKS, it was played in the Colonization rather than Conquest mode. The natives of the hexes to be annexed were put to the sword rather than swearing oaths of fealty, and then human settlers were imported. This was a slow and expensive process. Granted, the natives were beastmen and had no castles, but even beastmen are likely to prefer paying tribute and tolerating the presence of human farmers to extermination. I guess this might be another case of failing to play as Resource Extractors - we never really asked "can we get taxes and troops from your land without actually killing you?"
Notable exceptions to the Extermination Protocol occurred when faced with human natives, in three cases. Two were bands of nomad horsemen, who feared the party because many of their resurrected members made the horses nervous, and so fled their lands. The last was a band of berserksers, whose chieftain the party's top fighter maimed in single combat and subsequently took as a henchman. I'm not sure what became of his men but I presume that some use was found for them. Yet another reason I am not keen on beastmen for future use - they make aggression a very easy choice, and diplomacy an unattractive one.
Subjugating tribal / organized occupants of lands-to-be-conquered also sets up a known faction for later reuse. A small war between the subject tribes and tribes on the other side of the border might become the overlord's problem (since the tribes from the next valley over see the settlers as valid targets) or an opportunity (great justification for more subjugation). Wars between multiple subject tribes might threaten the realm's stability if they escalate, or conveniently weaken the tribes so that they can be integrated more readily. A coalition of subjugated tribes might rebel if they can overcome their differences (and a rival tribal coalition might offer to assist in suppressing the rebellion). None of this tribal warfare fits into the Monolithic State model we moderns are used to, but I expect it would make for a fine source of interesting intra-realm gaming, and one for which my playerbase is much better suited that courtly intrigue. This sort of "use what the tables give you" approach seems to have some support in the literature, too. Hell, we could do away with the d10-d10 civilian population growth mechanic entirely and cut down on the paperwork and agricultural investments while we're at it. Then switch thieves' guilds over to spy networks (you want to know what your tribal vassal leaders are up to, right?) and we're approaching a domain game I'd rather play than ACKS' default.
Also: in future, I'm totally going to try to make sure most mass combats happen at agreed-upon times and places, because resolving them otherwise, in messy circumstances not well-suited to formations, is quite a pain.