The Colosseum, also called the Flavian Ampitheater, is one of the great works of Roman engineering. In the 1st century AD, when it was built, it was the site of gladiator fights, mock sea battles, and other enactments. It had three stories of tiered seats and could hold 50,000 spectators. Large awnings shielded them from the sun. Underneath the floor were two levels of tunnels and cages, which were used to hold the gladiators and wild animals before the spectacles began. 

 Legend says that in the first three centuries of this era, Christian martyrs were thrown to wild beasts in the Colosseum, but there is no evidence of this. Most of the time, Christians lived peaceably in the Roman Empire. Organized persecution of Christians was only sporadic, and tended to happen when there was political unrest in the Empire. 

Today, the Colosseum is one of the most frequently visited monuments in Rome. The ticket is combined with tickets to the Roman Forum and Palatine Hill, and in early 2012 costs 12 euros. The ticket can be used for one entrance to each of the three sites, over two consecutive days. If there is a special exhibit at the Colosseum, a 3-euro supplement may be added. There are various concessions for citizens of the European Union, including free entrance for EU citizens under age 18 and over age 65. Full details of prices, as well as links to the official ticket sale site, and opening times, can be seen here:

http://www.coopculture.it/en/colosseo...

In the summer and at other busy times, there are often very long lines to buy tickets at the Colosseum. Buying a ticket from the official site allows you to avoid the ticket line. The lines at the Roman Forum and Palatine Hill are usually much shorter, so you can also save time by buying your ticket at one of those sites, since all three are included in a single ticket. Finally, if you plan to visit other museums  or  archaeological sites in Rome, and if you'll be using public transportation, the romapass may save you money, and will also allow you to bypass ticket lines:

http://www.romapass.it/p.aspx?l=en&tid=2

However, compare the cost of the pass to the cost of the museums you hope to visit, considering also how much you'll use the three-day transport pass that's included, before you decide to buy the pass. 

The Colosseum offers guided tours in Italian, English, and Spanish. When conditions permit, there are tours of the underground area. These tours are generally of high quality, and allow you to visit parts of the structure that ordinary ticket holders can't enter. Private tour guides also conduct tours that can visit these limited-access areas. In winter, the underground area is subject to flooding and usually can't be visited. There are excellent explanatory signs in Italian and English inside the Colosseum, so a tour is not really a necessity for understanding the amphitheater. Parts of the underground area can be seen from above without actually taking a tour. Many group tours combine the Colosseum in a single tour with the Roman Forum, but the Palatine Hill shouldn't be overlooked. 

 For choosing a tour guide, make sure to:  

  • Ask questions in advance and see if they are helpful from the beginning. Perhaps even make a phone call. 
  • Make sure to find out how large the tour groups are.  You may want a smaller group but be aware that you will pay much more for this.  The groups for the underground tour are very small since the area is small.  
  • Make sure your guide speaks good English (or your preferred language).  This can be a major deciding point in whether you enjoy your tour or not.
  • Find out if the company has tours that are catered to children or if the tour is recommended for children. (Also check for special child rates).  Most official tours won't cater to children although children are more than welcome on them.  Anyone under 18 will get free admission to the colosseum, forum and palatine hill.  They will only pay the additional cost of the tour.  If booking with a private company be sure their rates reflect this.
  • Check to see what past guests have said about the tour company.  Tripadvisor is a great place to start.
  • Don't choose a tour company whose main attraction is that they pre-purchase tickets. This will not help you at all as it only takes a a couple minutes to purchase your tickets with your guide and you won't have to pay any extra fee.
  • If you or a member of your group has physical difficulty walking you may want to use a wheelchair. Believe it or not the Colosseum does have an elevator. It can be difficult to bring a wheelchair thru the Roman Forum. 
  • Have a wonderful time at the Colosseum and give yourself plenty of time to explore!