[Dungeon] Chimer Stronghold

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DUNMER STRONGHOLD

Purpose: to quote a Bethesda developer, Dunmer Strongholds are:

GT Noonan wrote:
Best Westerns. The were basically fortified stronghold/checkpoint/hotels for travelers. There are no records of any of the strongholds ever being held under siege or used in any battles/wars, but it is quite possible that they were used for warriors as layover posts while travelling.

At least one of them (Kogoruhn) was apparently associated with a House (House Dagoth) while still in use. (I don't have any quote at hand at the moment, though).
Note that a Dunmer stronghold is also the setting for The Importance of Where, and the publisher's note provides some fairly useful information.

VVARDENFELL:

Purpose: aside from their purpose as described above, the strongholds on Vvardenfell also form a circular travel network via their propylon chambers. While the propylon network may have been used to quickly transport warriors to key locations, for example in the war against the Nords, each stronghold only appears to have a single associated propylon index, which would imply that only one person would be able to use any single propylon chamber at any time. (Unless there were more indexes originally and only one of each managed to survive, but that idea strikes me as silly). They may have been messenger relays.
Is there any pertinent information in Bethesda's propylon index mod? I frankly never got around to playing it.
Note that a few strongholds appear to have had unique purposes, such as the arena of Rotheran and the settlement of Valenvaryon.
The current inhabitants of the strongholds are typically either associated with Daedra or the Sixth House.

Distribution: 11 strongholds in a very crude ring around Vvardenfell. They tend to be close to water, though as the Inner Sea did not exist when they were built that can probably be chalked up to coincidence. (More on this later).

Layout: Andasreth is a good example of a typical Dunmer stronghold layout.

All strongholds have a rectangular base. Almost all of them are built on top of large platforms or 'pylons'. Two thirds of them have keeps, usually 'L'-shaped. All have an ashpit which is usually located on the roof of the keep, if there is a keep, and all of them have propylon chambers.
A hut is always located on the roof of a keep which leads to the 'Upper Level' of the keep interior. Keeps also have a 'Lower Level', which usually has one exit on the side of the keep.
Walls and domes are fairly common in the exterior. A few strongholds have sewers.

Entering the keep from the roof, a small stairway leads to an outer corridor that follows the contours of the keep walls. Alcoves (in_stronghold_corr2_06) dot the outer hall and are often grouped with shutters (in_strong_shutter00) which communicate with arrowslits in the exterior. The empty space contained within the outer corridor is filled by small, generally rectangular rooms, which typically communicate directly with the outer corridor; as well as the staircases to the lower floor and roof.
In most strongholds the Lower Level follows the same principal as the Upper Level layout, but its outer hall is often slightly larger. (Due to the slight slope of the keep's walls in the exterior). The interior often descends below the keep, sometimes connecting to an additional Underground cell. Aside from the keep, interior layouts vary widely, with the following exceptions:
The propylon chamber always looks the same, with two propylons in an otherwise fairly empty room.
Domes use a specific shell. (in_stronghold_dome00)
The huts in Valenvaryon also use a specific shell. (in_stronghold_roofext00 capped with in_stronghold_wall10)
Often, rooms are further subdivided by arches (in_stronghold_divider00 et al). Such rooms are especially common in lower levels. Hlormaren is filled to the brim with them, and as such makes for a good reference.
Very important: the narrow corridor assets in which one wall is sloped are only used for the outer corridors of the keep. The sloped wall is always on the outside, following the contours of the keep. In all other cases, only the straight-sided corridors are used.

Exceptions:
Berandas lacks the large platform, but it looks as though its platform may just have been buried under the soil and, likely, ash. The only really unusual thing about it is that its propylon chamber is on top of its keep rather than adjacent to it.
Falensarano's keep is only one story high, so the typical Upper Level and Lower Level are merged.
Indoranyon is a major exception. It lacks a keep and its entrance "has some sort of enchantment on it". According to UESP its interior was "magically altered", and it looks unlike any other Vvardenfell stronghold. It also contains a statue of Mehrunes Dagon.
Rotheran lacks a keep. It is an arena, with a non-standard interior.
Telasero lacks a keep. Its interior is also non-standard.
Valenvaryon lacks the large platform and keep, as well as the standard interior. Instead, it has a statue of Malacath. It also appears to have been a settlement; it is the only stronghold with square huts. It is, however, on top of an unnatural rectangular plateau, so, as with Berandas, it is possible that the large platform is simply buried.

MAINLAND

Purpose: TR's strongholds are currently not linked up to a propylon network.
Many of TR's strongholds -- such as Rift Pass -- have specific purposes in the vein of Rotheran. In general, given the scarcity and individualism of Dunmer strongholds in TR, it is hard to nail down any conventions:

 

TR_5-7_Red (-36, 19) - very small stronghold which mostly adheres to the Vvardenfell conventions, but with no propylon chamber, or room for one for that matter.

Velothi Mountains (-15, -5) - very small stronghold ruin buried halfway into a mountain. It has no propylon chamber, and its 'keep' has an unusual shape (half buried propylon chamber). It does, however, have an ashpit, and is otherwise similar to the Vvardenfell strongholds.

(Turen) Sacred Lands Region (40, -14) - Temple shrine with hostile warrior archaeologist(?) NPCs for some reason. It mostly follows Vvardenfell conventions, though its keep exterior and interior were sunken into the platform and the keep entry communicates with the 'Upper Level'. It has a dome on top of its keep.

(Tulesmath) Lan Orethan Region (33, -25) - lacks an interior, as such. Rather than a typical stronghold ruin, it appears to be a buried pass: it consists of a platform, a propylon building with its entrance blocked and what appears to be a sort of gateway communicating with the exterior in place of a keep. I think it might be interesting if this were a stronghold ruin under construction, work on it having been broken off, possibly by the eruption of Red Mountain or some other cataclysm.

(Baan Urlai) (Telmahre Cavern) Inlet Bog (19, -20) - a completely buried stronghold ruin complex. There's a shipwreck there which I am not entirely sure should exist, by the way. The most notable structure -- and the only one with an interior -- consists of a double platform surmounted by a small keep. The interior of the keep itself is a single large room, with stairs on either side descending into the upper of the two platforms.
The interior seems to be used by vampires, and access to it is blocked by a scripted cavern end piece by the looks of it. I am not familiar with the interior or any plans linked to it, so bear that in mind. Having said that, my proposal for Baan Urlai is that it was the capital of an early emerging House that may have rivalled Ebonheart and Almalexia in its day, but was likely buried with the creation of the Inner Sea, which also turned its once fertile territory bordering the Thirr into the Inlet Bog.

(Kogo'Bal)(Samursi) Boethian Mountains (36, 3) - another buried stronghold ruin, currently home to hostile mages (rogue Telvanni?) with (relatively) impeccably dressed slaves in a slave-pen (why?), the ruin itself consists of several buildings on top of a platform, including a low keep the interior of which is mostly caved in. This ruin could be a transitional stronghold-settlement much like Valenvaryon, perhaps the seat of a fledgeling House that never made it very far.

(Salandus) Alt Orethan Region (28, -26) - this fairly large stronghold is buried into the rockface near a Daedric ruin. Its exterior largely follows convention; like Berandas, the propylon building is on top of the keep, and the exterior also has a dome and an ashpit.
The interior, however, follows the pattern of TR strongholds, the propylon building being incorporated into the keep interior as a large room -- this one has an odd steeple-thingey going on with its ceiling -- and the interior also extending to the platform below the keep. It seems to be home to smugglers. It makes unconventional use of the slope-walled corridors, and has some odd high-ceilinged rooms. It also has a few slaves behind bars. Oddities aside, it still works as a pretty standard stronghold.

(Rift Pass) Northern Velothi Mountains (-31, 4) - a highly irregular pass structure which appears to have been built by Dunmer to protect the defeated Nords from their wrath. I have suggested removing the exterior and using the interiors as the basis for a new, more standard stronghold, possibly the former clanstead of St. Felms.

(Merihayan) Lan Orethan Region (33, -23) - a little north of Tulesmath, the exterior consists only of a bit of a platform and a propylon building sunk into a ridge. Much like Baan Urlai, the 'keep' interior is a large room, with most of the interior being in the platform. As with Baan Urlai, the sloped hallways are used for the platform interior rather than the keep interior. Aside from being close to Tulesmath, the ruins are right next to some Daedric ruins.

Bisandryon (30, -19) - unlike the other Strongholds, Bisandryon certainly looks fortified. Its keep occupies all of the visible platform space, the keep walls covered with equally spaced windows, and there is a dome and square hut on the roof. Aside from the keep, it shares some similarities with Rotheran, being built into the platform with straight-sided corridors circling a large central room in the interior, but its interior includes an indoor garden with artificial lighting and a 'war temple' with a shrine to each member of the Tribunal.
A separate keep structure leads to a separate, more ruined interior; first a large, open chamber with a broken bridge that once spanned it, and then below it another larger room with a vampire.
I think Bisandryon can be discussed later, as it is a rather high-profile location.

Distribution: taken from the book I linked earlier:

The Importance of Where wrote:
[...] few strongholds outside of sparsely-settled Vvardenfell have survived to the present.

There appear to be seven strongholds in TR_Mainland. As far as I can see, Maps 3-5 currently add three more, which includes Rift Pass. That being said, I can't speak for Uld Vraech given its incomplete and unmerged state, nor can I speak for the countless unmerged interiors. (I happen to know that the currently unused Velothi tower interior i4-414 uses some stronghold assets in its lowest levels). TR appears to have as many strongholds hidden in interiors as it has exposed strongholds. (And even the exposed strongholds are only partially exposed). That could be a good thing.
I wouldn't mind seeing a few conventional strongholds half buried or otherwise hard to access (having a sunken stronghold or two in the Inner Sea could provide some -- in my opinion -- much needed underwater exploration) with exteriors like Kogoruhn or Berandas. The total should probably stay in the single digits across the mainland, though. For the most part having small sections of stronghold hidden deep within other dungeons seems like a good way to use those assets.
I also wouldn't mind seeing two or three stronghold settlements like Valenvaryon, perhaps inhabited by Ashlanders or vagrants or critters.

Layout: of the seven strongholds in TR mainland, one (Kogo'Bal) mostly follows conventions (with the exception of its inward-facing shutters), and another (Turen) more-or-less follows conventions.
TR does tend to marginalize the keeps, often omitting them, making them very small or low or both or simply using the propylon chamber mesh instead. Instead of an outer hall with interior chambers, the keep interior often consists of one large room. Most of the interior, including the sloped outer walls which are generally only found in the keep, is instead located within the platform. Merihayan serves as a good example. We could run with that tendency and create a distinct Indoril stronghold style or something. (Most of TR's strongholds are located in traditionally Indoril land, from what I can see).