The War Games are battles that are played by the Roman demigods on the Fields of Mars. The forts that are built can be taken down and reconstructed in about an hour, according to Hazel Levesque a camper who resides at Camp Jupiter. Hazel explains that the Romans have to be prepared if their enemies were to attack them. She also states that the entire camp would take about 2 days to take down and move.
When Percy Jackson arrives at Camp Jupiter, he is shown the Fields of Mars. Hazel Levesque tells him that the War Games are held there. Percy took part in his first War game that night, and helped lead the Fifth Cohort to victory.
This is one of the many games that the Romans play, similar to the game "Capture the Flag" at Camp Half-Blood.