In Morrowind, Dunmer Strongholds are typically large stone buildings that are scattered around the land of Vvardenfell. Dunmer fortresses were originally built to fend off Nord incursions in an era before the Great Houses coalesced. Most of the Strongholds were abandoned by the original owners after the Tribunal Temple established itself as the major religion of the region. Most of the Dunmer Strongholds, although abandoned, are inhabited by various bandit and slaver groups, although some of the more isolated Strongholds have been occupied by higher risk groups such as House Dagoth and the Daedra.
Strongholds, after being abandoned, have been used for a wide variety of things, such as Military bases (Kogoruhn) to places were slavers reside with their merchandise (Hlormaren). The space available, and the shape of the strongholds makes them ideal hiding places for outlaws due to their easily defensible spots and most of the Strongholds being located in very isolated or obscure areas of Vvardenfell.
Most of the strongholds contain Propylon Chambers, which require an index to operate. Oddly enough, Kogoruhn, being both the largest and the most powerful stronghold, lacks the propylons themselves, and thus cannot be fast traveled to in any way. The indexes themselves, are scattered all over Vvardenfell. Assuming you have the correct index, you can fast travel between the two strongholds nearest your own; however, most strongholds are quite distant from any civilized areas, and this form of fast travel is really only somewhat helpful in reaching obscure regions of Vvandenfell's interior and extremities. One or two do feature friendly NPC's who occasionally offer training. (Secret Masters).
The Strongholds, themselves are usually quite profitable to a player, due to the wide variety of items located in most of them (Potions, scrolls and new weapons for example.) making the loot good for accommodating a wide spectrum of play styles with the loot being useful to all play styles.
Occasionally though, the Strongholds are targets of quests; at some point during quests, you could be sent to one of the following: Andasreth, Berandas, Indoranyon, Rotheran and Telasero. The quests vary from killing a high priority target to recovering an artefact, although occasionally, the two are grouped into one quest (Such as in the quest Slay Dagoth Tanis