Singing Towers

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

The Doctor also met a past version of himself on this 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". (TV: Last Night)