It's a recently released "old-school" RPG. (Stated inspirations are Darklands, Fallout, and Arcanum.)
Tom Chick did a decent review here: http://www.quartertothree.com/fp/2015/10/12/it-was-a-dark-and-stormy-night-in-age-of-decadence/
You can get it on Steam; it's currently on sale. http://store.steampowered.com/app/230070/
I mention it here because its setting, like the Auran Empire, takes more inspiration from the world of Late Antiquity than it does from the traditional medieval Europe thing. "Post-apocalyptic low-fantasy Rome" is a recently rare setting conceit, so I am sure that ACKSers could get some inspiration from Age of Decadence if they wanted to.
And, oh good lord, Darklands. Hands down the best computer RPG put out in those heady days, even more so that it was from Microprose, king of the simulators, and I'll internet-fistfight whoever says otherwise about either of those two statements.
A real sandbox romp through medieval Germany. Wilderness encounters galore; an "end game" that you could complete and just keep on playing afterwards. Characters aged; you'd retire them, bring on new people, equipment needed mending, practice made you better at things, etc.
I really need to revisit translating it for ACKS; the praying for aid from saints and the alchemy and all that. I think with that and Kiero's MLT work there's something really nice in there.
GOG has it for $5.99, without the Steam overhead. I have it from the original purchase, and a copy from the dark times before GOG and such.
Oh yea - I have Mount & Blade, and it is most definitely very ACKSy. Troops, henchmen, domain rulership, diplomacy. It's like an action movie version of ACKS. I usually call it "Grand Theft Medieval" though, cause it's just satisfying plowing through a bunch of mooks on a horse with a lance/sword and then taking their stuff.
I was actually a little overzealous in my first/current game because I totally wiped out a side instead of letting them sit and stew in a castle - I think it's overcomplicated the diplomatic scene to have lost the side.
I've carved up half of...the purple side and I killed off ...the green side and I now own their stuff. Can't be bothered to look up the names.
I never managed to so much as hold a castle for very long, not sure what I did wrong on that front. Would have loved to get to a kingly level and have absurd death stacks. Maybe it was the mods I was using (florian mods iirc) which made things more complicated and possibly harder.
Crusader Kings II is the shit, although for my part I came to like Europa Universalis IV better. CK2 takes place, in the base game, between 1066 and 1453, from the invasion of William the Conqueror to the fall of Constantinople. You can pick any ruler in europe of at least Count rank up to Kings and Emperors, and then you play a remarkably complex domain management game. You need to keep feisty vassals in line, secure alliances by marrying off your daughters, and attempt to groom your heir, who you will play as when your current character dies. Expansions add things like the ability to play as Muslim rulers, Merchant Republics, access to India, and some expansions move the selectable timeline back (Old Gods to 840, Charlemagne to whenever Charlemagne was around).
I’m in much the same boat - catastrophically bad at Mount and Blade (though it was fun), thought CKII was OK (good premise, but a bit overwhelming with the sheer number of characters you sometimes need to consider), and derived substantially more enjoyment from EUIV.