President Obama marveled at the sheer size of the Colosseum during a private sightseeing tour Thursday night in Rome.

"This is bigger than some current baseball stadiums," Obama told reporters, noting the size exceeded Wrigley Field in his hometown of Chicago. "I have to check on what it seats. It's remarkable. Unbelievable."

The amphitheatre, located in central Rome, was constructed between 70 and 80 A.D., and was estimated to hold between 50,000 and 80,000 spectators. The Colosseum was home to gladiator battles and animal hunts during the Roman Empire.

Obama walked along a platform running through the center of the arena, and observed the maze of tunnels that ran beneath the floor. 

He continued on to the third ring of the upper level, where he quizzed his tour guide about gladiatorial battles.

"Let's say someone is on the ground. He didn't have to kill them, did he?" the president asked.

The tour of the ancient ruins capped a lengthy day for the president, which included meetings with Pope Francis and other senior Vatican officials, as well as the Italian Prime Minister Matteo Renzi. On Friday, he'll fly to Saudi Arabia for the final leg of his weeklong trip.