The Pantheon in Rome was not built in a day. Twice destroyed and twice rebuilt, Rome's famous "Temple of All the Gods" began as a rectangular structure. Over the course of a century, this original Pantheon evolved into a domed building so famous that it inspired architects for the next 2,000 years.
Archeologists and historians debate which emperor and which architects designed Pantheon we see today. In 27 AD, Marcus Agrippa, the first emperor of the Roman Empire, commissioned a rectangular Pantheon building. Agrippa's Pantheon burnt down in 80 AD. All that remains is the front portico.
Another Roman Emperor, Titus Flavius Domitianus, (or, simply Domitian) rebuilt the Pantheon, but it burned down in about 110 AD.
Then, in 126 AD, Emperor Hadrian completely restored the Pantheon in Rome. This Roman Pantheon survived many centuries and wars. The Pantheon remains the best-preserved building in Rome.