Spirit of Tomorrow was an arcology, a massive architectural residence located in Red Square in Moscow during the 23rd century.
History and specifics[]
In the 2250s, when Pavel Chekov visited the Red Star arcology as a boy, he wrote in his diary that it was taller than Spirit of Tomorrow. (TOS reference: Star Trek II Biographies)
In 2277, while under the influence of an illusion-generating machine, Pavel Chekov saw Spirit of Tomorrow in Red Square, along with three nearby arcologies Kalilingrad, Leningorá, and Red Star. (TOS - Plot-Your-Own Adventure Stories novella: Distress Call!)