(GMT)08 SEP 2016 COLT333:
The Colosseum is unbelievably huge and has so much history in the architectural details. The lines to buy tickets are long, so buy your tickets online! you can buy a combo ticket with the forum.
It can be used on the selected date at the time you prefer according to the availability of the Colosseum (max 3.000 visitors at once inside the site)