Perhaps the most famous sight in all of Rome is the Colosseum. It is a very famous arena in which gladiators fought, in addition to other types of violent activities. There are other Colosseums around the Roman Empire, but the one in Rome is much larger than the others. It has been partially destroyed by invaders, but you can still see how gigantic the structure was.
Another interesting monument in Rome is that to the first king of unified Itally, Vittorio Immanuel II. His monument is very controversial because it is very different from the other buildings of Rome. First of all, it is much whiter than the old, brown monuments from the Roman Empire. Second, it is much taller and more square than the buildings around it. Because of this, people often refer to it as the "Wedding Cake" because it looks like a gargantuan, white wedding cake. And lastly, an ancient Roman temple was destroyed to make room for the monument so many citizens were understandably upset about that.
The Trevi Fountain is one of the most beautiful fountains in the world. It is carved into the front of a very angular building, but the fountain was made to look as if it were more organic than its attached edifice. There are horses and gods and waves all winding away along the fountains smooth lines. It is tradition to make a wish and then throw a coin over your shoulder into the fountain to ensure that it comes true. Or maybe that's just to collect more money from tourists. :)
A little further away is the Roman Pantheon. This building was made to house the Roman gods, but today it has tombs and Christian statues. The dome of the Pantheon is much larger than other domes of its style. In fact, it is the largest concrete dome in the world. A sphere with a 90-foot (27.4 meter) diameter could fit inside of the pantheon. That is how large the dome is! The Romans were so good at using concrete that once the Roman Empire fell it took the world over 1000 years to perfect its use again.
Vatican City, the seat of the Catholic Church, is in Rome as well. It is a very beautiful city/country with many famous works of art. St. Peter's Basilica is the most famous building in the vatican. It is much nicer than my old church at home. In fact, it is one of the most impressive buildings I have ever been in. There are letters near the top that appear tiny, when in fact they are over two meters tall! People gather in the square out front to hear important announcements and for religous holidays. The square long and straight, but has a a circular portion that is a little bit wider.
If you ever get a chance to visit Rome, you should! It is my favorite city and I love it very much. And make sure to get gelato from Giolitti's, near the Pantheon!

