Ruins of Bthalft are Dwemer ruins located south of Ivarstead.
Some mead barrels and a Dwemer Convector, used for Arniel's Endeavors, can be found here. A corundum ore deposit is located on the eastern edge, and there is also a small camp on that side, presumably used by the resident bandits, where a locked chest can be found.
With Dawnguard installed, a tower can be raised after finding all four Aetherium Shards during Lost to the Ages. It gives access to the Aetherium Forge.
 Related Quests
Some radiant quests have a sizeable list of possible locations. These quests are not placed on individual place pages, and are instead listed at radiant quest locations