Brilliant indeed,
Nerwen, music, or songs, is the answer, though
Galadriel55 has found the three couples. The descriptions were supposed to be taken litterally for each one of them. Friend behind closed doors in the highest tower, lover in deep dungeons under wide waters and kinsman enchained in the mountains.
So I guess the thread is
Nerwen's now, though I know you used to be
terrible at this.