Create a Gold Mine- that gives mostly gold ore or stone... nothing else... and make sure it can't proc around any existing fortification nor can any substantial fortification be built around it...
Put up an fixed structure that can be minimally improved (corners that are 3-4 spaces apart that can't be extended, etc).
Since Metal walls came out- everyone has been fortifying like mad; but that only creates complicated, extended fights over likely nothing (Junction City is prime example) by the time you sacrifice time and effort to get in... pickings are slim indeed.
How about a resource everyone wants... but nobody can really claim without trial by arms 24/7? Essentially a no scents zone...?
I'm imaginging an area (or several around the map) with scattered Ruins of a sort, with Mine's that have a chance to produce Gold with each Season (like the random events), but also with a chance to build additional Mine Entrances to find better?! Need to make Mine Entrances decay-able in this 'area'...
Bar portals within the area and no claims?!
Two Cents