"The adventurer must now secure his domain by establishing a stronghold on its territory. If there is an existing structure that’s suitable for use as a stronghold on the domain, this structure can be claimed by the adventurer as his stronghold. This structure might be a castle granted with the land, or a tunnel complex cleared of its prior inhabitants in the process of securing the domain, and so on."
Strongholds and Domains, AXIOMS 3
Since a 10' x 10' x 10' dungeon corridor costs 500gp and has 80 shp, I assume a constructed dungeon has a stronghold value equal to the number of 10' squares in the dungeon x 500gp, and a number of structural hit points equal to the number of 10' squares in the dungeon x 80 shp. However, the 10' x 10' x 10' dungeon corridor has no unit capacity.
So, how does a dungeon stronghold work in a siege or assault? Do you use the shp / 1000 abstraction formula to determine it's unit capacity? It seems off to be able to have as many units attacking a dungeon with one entrance as you can have attacking a sprawling fortress on the surface. On the other hand, if dungeons make better strongholds, then why doesn't everyone use them?
Also, how does the stronghold value and shp of a natural cave system compare to one that was constructed by men, or by dwarves?
Unit Capacity for defenders of dungeons ("Dungeon Defense Unit Capacity") is 1 unit per 24 10' x 10' squares.
Unit Capacity for attackers of dungeons ("Dungeon Assault Unit Capacity") is equal to the number of entrances to the dungeon plus the number of breaches.
The number of units required to blockade a dungeon is equal to twice the Dungeon Assault Unit Capacity, with a minimum of 2 units required.
Siege-mining can be used to directly create additional dungeon entrances instead of dealing SHP damage. Each 6,250gp of mining creates one additional dungeon entrance. (Breach = 1000shp; 10'x10' equals 80shp; 1000shp/80shp=12.5; 10'x10' equals 500gp; 12.5x500gp=6,250gp.)
The net result is that dungeons are much easier to blockade but much harder to assault. You would not want to use a dungeon to defend a high-value location with good supply routes - the enemy will blockade you and starve you out. Using a dungeon located in an out-of-the-way area, however, would mean a blockading army would face its own supply challenges, and likely it wouldn't be worth an extensive blockade. Conversely, assault dungeons is the province of elite units with high battle rating that can fight their way in - or of adventurers, functioning like special forces.
For ACKS purposes I imagine that the ideal stronghold is one that has an above-ground component and an underground component.