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 Husbands of River Song)
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. (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 twelfth incarnation, as he kept putting it off. In the aftermath of a spaceship crash on the planet, the Doctor told a man searching for survivors to build a restaurant on the exact spot in front of the Singing Towers, and later attended dinner there with River. The Twelfth Doctor gifted River a sonic screwdriver and they spent their final night together, which on Darillium was 24 years. (TV: The Husbands of River Song)