The BBC's Guy De Launey in Phnom Penh says the Baphuon was once among the finest of the great monuments of Angkor, but by the 1950s it was on the brink of collapse.
A French-led team of archaeologists decided that the only way to save the temple was to take it apart, our correspondent says.
They dismantled the monument, laying all the stone blocks in the surrounding jungle. Each piece was painted with a number, matching an entry on the master plan, so the tower could be rebuilt.
But work was disrupted by the civil war and the records needed to reconstruct it were destroyed by the Khmer Rouge, a hardline communist regime that took power in 1975.
The reconstruction was only restarted in 1995.
Pascal Royere, who has been overseeing the project, said the early years had been the hardest.
"We were facing a three-dimensional puzzle, a 300,000-piece puzzle to which we had lost the picture. And that was the main difficulty of this project," Mr Royere told AFP news agency.
"There is no mortar that fills the cracks which means that each stone has its own place. You will not find two blocks that have the same dimensions."
Kudos to those who made this happen, both French and Cambodian!