When ambling around the central part of the Old City, you'll likely happen upon the Pantheon unexpectedly unless you're specifically looking for it. It somehow doesn't seem fitting that Ancient Rome's most well preserved, intact building is all but hidden and obscurely swallowed up by it's environs. But once you've found it - I consider it second only to the Colosseum!
There's free admission with opening times from 9:00 a.m. - 6:30 p.m. making it easily accessible for as many times as I couldn't help but pass through this central area. Seeing the Pantheon at all hours of the day is one of the added bonuses thanks to the oculus - the circular opening in the dome that provides the cavernous interior with natural lighting. Depending on the time of day, the sun casts its light and prisms across this suprisingly colorful setting which is likely Rome's greatest showcase for the use of differently toned marbles.
Hadrian's revamping of Marcus Agrippa's original from 27 B.C. is deemed "highly innovative" and one of the greatest architectural wonders to ever be accomplished in Ancient Rome. Guidebooks list the nitty-gritty of dimensionals and technical terms, but nothing suffices standing in the vast openness and looking up, all around.
Thankfully, the Pantheon survived because early on it was given to the Christians after initial construction was to house a place of worship to all the Roman Gods...and all such structures were evil and basically wiped from the faces of maps. There's some amazing accounts of what plunders and pillaging have taken place over the years - including that the Vatican's St. Peter's canopy over the main altar is made from bronze that was taken off the portico and melted down.
Considering Rome's abundance of churches/cathedrals, I wouldn't compare this in any way, but it certainly commands a silent reverant response from the massive crowds passing through; guards onhand to insure it, also! Aside from the art and architectural designs, the Pantheon is likely most known for housing the tomb of Raphael; the famous painter who contributed so much to the Vatican, Rome and the world. Found it also kind of ironic that King Victor Emmanuel II is also encrypted here - like the monstrosity of a monument he created unto himself wasn't good OR big enough!
The Pantheon earned more personal repsect for Hadrian at the top of my list for favored Emperors. Legacies may have passed, but his architectural contributions can still be seen ALL around the former Roman Empire; here was/is his best! Just east a couple blocks on side streets are columns from his Temple which have been incorporated into the Rome Stock Exchange building.