What is Velothi in The elder scrolls?

[Anantram]

Velothi is another name for the Chimer who followed the prophet Veloth out from Summerset Isles to Morrowind. These were the ones who later became the Dunmer. Thus, Velothi architecture is the architecture of the early Chimer. The towers are not temples, just built in the same style. As for them not being "towers", I don't think buildings necessarily have to be [censored]-shaped to pass as towers. The strongholds were heavily influenced by Dwemer architecture, when you look at them. They're making with stones what the dwemer did with metal.

[Beothuk]

As for why a same people would use different architecture styles for strongholds and houses, gee, I don't know. House Dres' architecture has not been defined by Bethesda. It's one of the two houses that aren't in Vvardenfell, and Tribunal showed us what House Indoril's architecture was like, so only Dres is left out. You can get a small peek at what it could have been in the Redguard comic, as Cyrus briefly went to free some slaves in Dres lands.

[Delaware]

You seem to be missing the point. The "factions" you mention either do not have any sort of cultural influence, or have only recently moved into Morrowind. The Chimer came to Morrowind as a group of nomadic tribes, that settle down and over the years merged into the Great Houses. Unless you think they are still living in Yurts, they will most definitely have gone through some architectural changes. Tilesets are unrealistic in the sense that nobody builds exactly the same all the time and an inability to mix different styles without obviously clashing. So the argument that it is unrealistic for each faction to have their own architecture is trumped by the already apparent unrealism of tile sets. So if there must be tile sets, then every faction ought to have his own.

[Erie]

It being a slaver camp says nothing about it being a temporary or permanent camp. The lack of real building is because they're slaves. Decorating a wall with bas-relief and other luxuries takes time and is totally useless if you don't plan on staying there for a long while. Your theory on shells is disproven by their conspicuous lack in ashlander settlements. If you remember the Redoran Stronghold Quest then you might consider the response of the architect and others when discussing the nature of that architecture - in the Ancient style or some such.

[Fox28]

Different sects can and do develop their own architecture (or several as is the case), but when a sect or religion takes over a pre-existing building, the old architecture persists. Istanbul, Rome and Delft (to name but a few examples) are full of examples of this. This doesn’t falsify your statement, of course. Both are equally true. I don’t know exactly to what extent this has been implemented in Morrowind, though. I looked it up on UESP a while ago but it only came up with Ald Velothi. If I am not correct, Velothian Architecture is just house dress stuff right? Or is it something else. Also, Velothi towers are not towers, just temples, so why are they called towers. Velothi is a style. Make-forts-with-the-stuff-we-got another. It's a generic Chimer Dunmer style and it includes the cantons of Vivec.

[Gajendra1]

It wouldn't surprise me if House Dres did use Velothi architecture. They are notoriously conservative, still worshipping the three Good Daedra alongside the Tribunal in the manner of the Chimer. Sticking to traditional building styles with proper Boethian architecture would seem right up their alley. Not that I don't think they could have developed their own I'm just saying I wouldn't be surprised if they hadn't.