Singing Towers

The Singing Towers of Darillium was where the Doctor spent what he knew would be River Song's last night with him. (TV: Forest of the Dead)

The Singing Towers got their name from the fact that the wind passing through the cave system on Darillium would harmonise with the cave's crystal layer, creating melodic tunes. When talking about them, the Doctor remarked, "They've been there for millions of years, through storms and floods and wars and... time. Nobody really understands where the music comes from. It's probably something to do with the precise positions, the distance between both towers. Even the locals aren't sure. All anyone will ever tell you is that when the wind stands fair and the night is perfect... when you least expect it... but always... when you need it the most... there is a song. (TV: The Husbands of River Song)

The Eleventh Doctor met a past version of himself on what he thought would be that night, when River went into the wrong version of the TARDIS. The younger Doctor asked his future self if this was to be the last time he would see River; the future Doctor replied, "Spoilers." (HOMEVID: Last Night)

However, while the Eleventh Doctor had planned to take River to the Singing Towers, he did not actually experience their final night until his next incarnation, as he kept putting it off. In the aftermath of a space ship crash on the planet, the Doctor tells a man searching for survivors to build a restaurant on the exact spot in front of the singing towers, and later attends dinner there with River. The Twelfth Doctor finally gifts River the sonic screwdriver that she will possess in Silence In The Library, and they spend their final night together, which on Darillium is 24 years. (TV: The Husbands of River Song)