You can not get the key until after you have the battle tower. At that point you can get into the quarry tower in Broken Valley (near where there was a cutscene when you first approached, with the miners blowing up the main entrance), and head upstairs to rescue the guild master. Once you have the dragon form, Lovis' tower and the quarry tower are pretty easy to spot from the air.
In D2:ED the quarry tower was open from the start, but the large jump in opponent level made it difficult to get the key early (though not impossible if you were not too concerned with protecting the miners after you got the key as they tried to escape).
You will want to finish up everything you want to do in Broken valley, the fjords and on Sentinel Island before heading to the Hall of Echoes, so there is no rush to complete the associated quests. Likewise, if you do not have the battle tower yet, you will want to finish everything that you can do in Broken Valley (except getting the mine key) and on Sentinel Island (except retrieve some loot from a boat in a tree) before doing so.
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