Rome is crammed full of so much history you'll need some help from a guide. The following tours are places I've relished visiting when I'm in Rome. Many you can see by just walking around. But sometimes an organized tour gives you a bit more time to learn about things rather than fumbling with maps and guidebooks.
Vatican City (the world's smallest country) and Sistine Chapel
Colosseum and Roman Forum
Galleria Borghese (for art lovers)
For ancient history buffs, add the National Etruscan Museum.
For major Roman Art attractions we recommend The Tour Guy tours. They know the art. They also know that statistics and facts can sometimes overwhelm. The tours achieve a balance between intelect and fun. Walkers will enjoy the Walks of Italy tours. Scholarly folks would like the Context Travel private and small group tours to Rome.
Castel Sant'Angelo - While it isn't on many people's Must See list, its a fascinating collection of re-used space. Starting as Hadrian’s Mausoleum built by The Emperor in the second century AD to house his tomb, it became a Papal palace with a secret passageway to the Vatican. There were prisons and torture chambers. Finally, it became Army barracks. Today the whole deal is a museum, and best off all perhaps, you can get cheap eats with a fantastic view of Rome and the Tiber. You can do it alone or hear about all the castle intrique recounted by knowlegable guides. That's an apartment just below. Nice, eh?
Trevi Fountain (although for a small charge we recommend the underground City of Water, which is where all that water goes you see flowing in the fountain.)
The Appian Way (if you walk, otherwise there's bus fare)
If you'd like to take an easy guided tour of the ancient city of Ostia, which certainly rivals the better known archaeological sites like ancient Pompeii, the Ancient Ostia Small Group Day Tour from Rome should fit the bill at a reasonable price. Ostia was once the ancient port of Rome and the city, founded in the fourth century BC, was abandoned when the port silted up. Great preservation of the buildings. And rather than being a resort, it's a real, working town--so you'll see the apartments of working folks just as they were thousands of years ago. [online tickets: Archaeological Park of Ostia Antica Tickets.]
You can also do it yourself and save about 30 dollars if all you want to do is wander around the ruins and read the signs. Here are the instructions for visiting Ostia on your own.
If you're in Rome and get bitten by the archaeology bug and want to visit the ruins to the south in Campania, You can take a tour of Pompeii and Vesuvious from Rome. The tour will save you from having to make train reservations and worrying about schedules. You can get some exercise on the trip by walking up to the summit of Vesuvius where you'll get a very intersting view.
If you want to really feel like you're in Ancient Rome, I suggest a walk along the Appian way and then ducking into the Catacombs when the weather begins to swealter. Nothing like inching along ancient hallways stacked with bones to remind you you're not in Peoria any more. Viator offers many ways to see the Appian way, from walking to Vespas. For tickets, see Appian Way Tickets and use this Appian Way guide to the treasures along this historic road.
David Downie, author of Quiet Corners of Rome, gives highly-rated tours of Rome with his wife, photographer Alison Harris. Make a beeline to those quiet corners, or anywhere else you'd like to learn about Rome: David Downie Paris and Rome Tours.
You know what's popular? The Rome Segway Tour. If you haven't ridden one of these contraptions, don't worry, you'll learn quickly. Even if you're as unbalanced as I am, these things are designed to make you stable, at least for getting around a city at a slow pace. That's why they cost a zillion bucks. You can pretend you're a celebrity zipping around Roma; other tourists, envious--or, maybe they're thinking you're a complete dork--will raise their cameras as one and you can't outrun the clicking of shutters as you glide down the cobbled biways of the Eternal City. Just do it.
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