Broken Valley Mine Tower is a location accessible while in dragon form after Damian's army invades the Broken Valley. It is located in the south-central area of the valley, southeast from Lovis' Tower, and northwest of Broken Valley Mine (see map below).
Upon entering the tower, a conversation is overheard between two Black Ring members, Scroud and Siddell. They notice you, so take them out quickly. Once they have been disposed of, it is time to loot your way up the tower. On the ground floor you'll find three chests, two of which are locked but can be lockpicked. Up a short flight of stairs to a wooden platform with winches you'll find two more unlocked chests. Proceed up to the next floor, where you'll find four more chests, and a small box under the stairs. Some of the chests can be lockpicked, but one requires a key. They all contain random loot.
At the top of the next flight of stairs you'll encounter Williams, Tanya, and Brent. Williams will provide you with a key to the Broken Valley Mine that is needed for the quest Lock and Key, and he can be mindread to learn about a treasure chest in the mine office. You'll automatically begin the quest A Guild Without a Master at the end of your conversation. Simply escort him to the ground floor of the tower unharmed, fending off the invading Black Ring members as you go, to complete the quest.
Return to the top floor and pick up the key from an alchemy table, it opens the locked chest on the floor below. Loot it, and you've done all there is to do at this location.