Orc Strongholds

Orc Strongholds are scattered throughout Skyrim. The strongholds are for orcs only, but you can be granted entry if you do a small quest for them.

Gaining entrance
When you approach a stronghold for the first time, a guard will stop you and tell you that this place is exclusively for orcs and outsiders will not be permitted inside.

However, if you ask how he/she can be persuaded to give you access, he/she will tell you that you have to reclaim an item, The Forgemaster's Fingers.

It seems though, that the Fingers's location is different for each stronghold, but you only have to do the quest once to get entrance for all of the strongholds.

Strongholds
There are atleast four Strongholds in Skyrim;
 * Mor Khazgur - Northwestern corner of Skyrim, straight north from Karthwasten.
 * Dushnikh Yal - Southwestern corner of Skyrim, straight south from Salvius Farm (Markarth ).
 * Largashbur - Southeastern quadrant of Skyrim, straight west from Riften.
 * Narzulbur - Mid-eastern part of Skyrim, straight north from Mzulft.