So let's look at some different classic settings and throw out some ideas. After that we will look at ways for even low-level PCs to join in the fun.
Greyhawk: This is quintessential stronghold-land. Lots of open space, political intrigue, warring factions, and beasties make this a great setting for traditional stronghold stylings.
Points of Light (4e): Rebuilding civilization, reclaiming lost territories, pushing back the wild unwashed monsters-PoL is prime real estate for stronghold rules.
Dark Sun: Now this could get interesting. A stronghold doesn't have to be a massive tower-laden structure. Maybe it amounts to smallish shelter for a fledgling colony of escaped slaves in Athas. Maybe the party has discovered a source of water or metal and needs a permanent base to exploit it. The construction could be threatened more by the environment than the local monsters. You could do something really cool with a pseudo-stronghold/barony here, something not unlike Bartertown from Mad Max 3. If I had all the time in the world, I would run something like this myself.
Ravenloft: If you can't beat em', join em'. Never going to escape that demi-plane? Make it your own. With the right group, it could be fun to make your own haunted manor. Gargoyles optional.
Eberron: This is such a wide, rich setting that there are all kinds of geographical areas where strongholds make sense. I think the Eldeen Reaches would be a great spot, but any "borderlands" would work. You could incorporate magic into the construction-maybe you hire Artificers as opposed to bricklayers.
Forgotten Realms: Regardless of the time period you prefer to work in, there is ample room to support a stronghold system here. I personally would love to do a Maztica campaign with stronghold rules, a "conquistador" type deal. If you haven't noticed, I have a lot of hypothetical campaigns in my brain that I will never likely have the time to run.
Spelljammer/Planescape/Astral Sea/Elemental Chaos: I lump these together only because they all involve unusual areas that are highly varied and mutable. There are all sorts of cool ideas you could use. Maybe instead of creating a castle, you are trying to set up a trading post in the Astral Sea. Or a Githzerai monastery on a floating chunk of earth. You are only limited by your imagination; don't be afraid to reflavor a stronghold into something else if it better fits the setting and/or narrative.
Hollow World: You could build an entire campaign around the idea of settling the interior of a planet. Could get very "Charlton Heston".
Birthright: No need to do anything! Has an awesome system already!
I hope this has your wheels turning a bit. My point in all of this is simply to show that any campaign setting, published or otherwise, can take on some stronghold rules if they are creatively shaped to the setting. It doesn't always need to be a knightly castle. It could be an underground hub in the desert, a floating chop shop, or literally anything else that requires an area to be cleared, settled, and built upon.
Ok, so I realize I haven't given ya'll the pricing for the strongholds and labor yet. That is because I am still working on it, as well as a new comprehensive list of mundane gear with new prices for use with inherent bonuses, and by extension, my magazine. Prices for mundane armor and weapons/implements stay the same as in traditional 4e, but I will have new prices for the rest of the gear that better fits the lower estimated wealth of adventurers. Anyhoo, we can still talk in the abstract about this next piece, which is looking at some ideas to allow low-level (read: broke) adventurers to still build strongholds.
Traditionally it was only high-level characters that were allowed/able to build strongholds and develop land. This was not just due to the cost restrictions; it was a reward for a PC's accomplishments. Anyone that ever played OD&D will tell you that hitting 9th level+ was indeed an accomplishment. I never got that far, except in one-off games. We died all the time. ALL the time. It can take years to get that much experience, and frankly, some DMs and players do not want to wait that long to do something cool.
One piece of DM advice that I think is priceless is "Get to the good stuff". If you hold back on your great ideas, you never know when your campaign will blow up. You could spend a year setting everything up to get to a moment that will never come to pass. So if you have a cool idea, let it fly. Strongholds could be viewed in much the same way. Rather than never making it to a level that is traditionally associated with stronghold creation, try these ideas for low-level play.
-The PCs pool their funds to build a group stronghold (not a bad idea anyways)
-The PCs re-purpose a dungeon or lair they have conquered
-The PCs are charged with helping to oversee the construction of one or more strongholds for a more powerful NPC, such as a king
-The PCs inherit or are rewarded with a run-down stronghold. The area needs to be cleared again, etc but they do not need to build from scratch.
Well, I hope that these ideas show a different way to look at stronghold systems. It need not be an Arthurian castle; it can be any structure, anywhere, that requires securing the surrounding lands and construction of some sort. You can also work around some of the costs to enjoy a stronghold system earlier in a campaign or adventuring career.
So, just to recap, I am working hard on the price lists. These will include construction costs, specialized hireling costs, and costs for mundane gear as an added bonus. After that is completed I will give you some generic rules for clearing areas, attracting settlers, running a barony, etc that will fall in line with old classic systems. Happy Friday!