On Stronghold Clearing & Reclamation

tl:dr; pulling apart a theme of 'taking over unoccupied castles' and managing realms above a parties level.

Working through my old campaigns I realise a strong motif that crops up again and again is the players at a low level get given the keys to some giant old fortress with mysterious locked up depths, residents of dubious loyalty and rumoured treasures. So far "sounds like a dungeon" you might say and true to a point. The key difference I have found to it all is that from day one the players nominally own everything they are working through and so are a lot more invested in not just smashing their way in and looting what can be easily carried away - they spend a lot more time and effort cleaning out and rehabilitating 'their' fortress.

Kraken Mesa, as appearing in chart if Styx flow through Gehenna

The things I love about this set up is -
1. It gives the players a lot of resources beyond themselves that are poorly understood and may be unreliable. They get fun choices of sending their 'minions' to deal with something and getting a uncertain result or go handle it themselves at the risk of their own skin.
2. They can engage with big problems, briefly punching way about their weight at the loss of some of their capabilities - parts of the fortress destroyed, minions slain or supplies sacrified to traps.
3. You can build a neat group of NPCs - the major residents - and their problems will become the partys problems.
4. Reclaiming, clearing or reactivating parts of the fortress are an endless driver for adventures - and 'treasures' are capacities added to the fortress rather than players themselves becoming near-godlike entities.
5. There is an additional layer of 'mini-realm management' where the party gets enhanced agency but also a whole slew of additional responsibilities.

First to run through the variations on this theme I have run:

The Gate of Ages: first of these was the north star of my first classic fantasy world campaign - the players were racing a demon-lead crusade to an ancient site from a long-gone empire of element worshiping ogres. They found a bunch of moth-balled facilities and a care-taking crew of doppelgangers. They withstood siege, used the facilities to carry out a full-body transplant of their party necromancer, then stole the keys from the dimensional lock in the basement and fled the plane.

Kraken Mesa: the second campaign was explicitly the archetype of this piece - the players were abducted from their prime, bundled away to Gehenna and tasked by yugoloths to clear out and set up an illithid fortress that they had been dragged to Gehenna. We got a solid academic year of campaigning from delving into the various wings and depths, finding out what factions remained after the shock of being hauled across the planes and reorganising everyone along the lines of 'we are all stuck here on the lower planes, we hang together or the Blood War will crush us all'. More tales from the Lower planes are here.

Thenya Palace: home campaign where the party were scions of the ruling house. They had call on all the resources of the realm provided they were happy to justify themselves to Grandfather-the-Duke afterwards. The party spent a big block of time clearing out and securing the once-sealed old halls of the palace. They also got good use out of the Palace Guard as NPCs and PC-proxies - sending the Guards to round up suspects, watch locations and also to serve as temporary PC's when the party split and one of the group (usually the Cleric) went off to do something hazardous without the others. A detailed encounter table for Thenya Palace is here, to show what is at what level.

Nowadays there are systems like Strongholds and Followers from MCDM (which I have not yet read) which give crunchy mechanics and additional abilities for players to use. This is a slightly different mode - one that gives players a power boost anchored to their holdings. This was not the model I used - I went more 'broad' in that your minions can probably handle 'more' relatively ordinary things, taking what UltraViolet Grasslands terms 'the milk-runs' off your hands and leave the party focused on the interesting stuff.

The key things I learned from all this -
Players get to do some fun complex challenges - juggling the abilities of the different types of minions, facilities and requirements can be a great source of adventure hooks.
Care has to be taken to balance the 'place of their own' draw against the associated hazard. Throwing a massive threat at the place first thing out of the gate may just lead to the party deciding 'easy come, easy go' and bailing.
The minions and assets under the players control should be useful but not too useful - otherwise it only makes sense to delegate any hazardous duty to Sir Joe the Competent and just have him do everything. My approach was always that the NPCs are generally baseline competent and will triumph over any mooks or low level foes they go up against but anything which requires creative spark will see them out-matched and either hurt or evaded.

The game has long had stronghold management as a high level aspect but there is lots of fun to be had with low level parties 'riding the tiger' of managing realms above their pay-grade too.