The stronghold's effective value is reduced by the unpaid upkeep. The stronghold's upkeep cost is not reduced, however. (Like a big house, it doesn't get cheaper to maintain by not maintaining it!)
A stronghold's value has implications for the size of the domain that can be controlled as well as the defensibility of the stronghold during sieges.
That said, "there is a lot of ruin in a nation," as Burke once said. Likewise, a stronghold can last a long time with little upkeep.
Lol. I had initially titled this post “How Much Ruin in an ACKS Nation?”, but decided to go with something that would show up in a search for “upkeep” instead.
Thanks for the answer. Does the same apply to urban infrastructure? There’s certainly a lot of ruin in Cynidicea, and a lot more fortification and infrastructure than it can hope to support.