What is the game they play at the pier?

Navy Pier is an old, abandoned part of the city with a carousel, a Ferris wheel, and many crumbling and empty buildings. It is located in an old part of Chicago, which is no longer inhabited.
Tris is a new Dauntless initiate, having transferred from Abnegation. During the first part of their training, the Dauntless initiates have to face a number of challenges in order to prove themselves worthy, such as fighting sessions. One of these challenges is a game of capture the flag, a Dauntless tradition. The initiates are divided into two teams; each team must hide their own flag and make it hard to find, and then find the opposing team's flag. The first team to find the other team's flag wins.
Tris understands that the Ferris wheel can be a great vantage point and allow her to see what the other team is doing, so she climbs up, followed by Four. She manages to see where the other team has hidden the flag and climbs down the Ferris wheel. She almost falls and is saved by Four. Once she gets down, she is able to tell her team where the other team has hidden the flag, and this leads to her team's victory.