Behold the Roman theater in Bosra, Syria, and consider that it was probably build during the reign of Trajan and is therefore more than 1900 years old. It is 334 feet across (102 m) and held 15,000 spectators.
Should the war ever end and you ever get there, you could walk through this tunnel into the theater, just as people did in the days of Hadrian, of Septimus Severus, of Constantine. As to how it came to be so well preserved, well, partly the local stone is just very hard and durable, but there is more.
As you can see in this aerial photo it was built into an Arab fortress that dates to around the 10th century.
"Around" is the best I can do; UNESCO actually dates the fortress to "AD 481-1250." Anyway the fortress walls protected the theater, helping to keep it standing. Meanwhile the interior of the theater was turned into a palace and filled with three stories of rooms, all built of stone. After those were abandoned in the later Middle Ages, the whole place filled with sand, which also helped with the preservation.
In 1946 a major effort was begun to excavate and restore the theater. Here you can see the ruins of the medieval palace within the theater.
The restorers were only interested in the Roman theater, and the medieval palace was all demolished and removed. Here is the theater at the end of excavation, before anything was rebuilt. By comparing this to the photo at top you can see that some of the stage was reconstructed, but not really all that much.