With the main entrance barricaded, you must find an alternate method of entry. Guildmaster Williams not only knows of a secondary door, but also holds the key. He can be found on the top floor of the mine tower, southeast of Lovis' Tower and northwest of the mine (see Broken Valley Mine Tower for details). After obtaining the key, take your dragon form once again and travel southeast to the entrance marked on the map. You'll have to destroy some air defenses (mainly ballista towers) to land safely outside the door.