From Diablo Wiki
Trigger: Speak with Sergeant Burroughs
Completion Details: Free the recruits so they can raise the catapult. / Protect the recruits as they raise the catapult.
The event is very straight-forward. Burroughs will accompany the hero roughly one screen north, killing some Demon Troopers and potentially Fallen as well to free the recruits. The recruits then move into position at the pulleys in front of a pit, and begin raising the catapult from below. It takes them approximately one minute to do this. If a recruit is killed, then the event will take longer.
Hellflyers and Troopers will spawn from the north, south, and from the cliff wall facing away from the pulleys. The Hellflyers will fly in along with them, usually landing on top of the recruits, so it's best to keep a post right on top of them and be ready for trouble.
A wave of monsters will appear around 25% completion, 50% completion, and 75% completion. When the catapult is nearly raised, a large wave of flyers will come in and the event is likely to be over before the flyers are dealt with. It ends once the red bar in the quest tracker is filled.
A video of the event:
|Quests & Events of Diablo III  Act I Act II Act III & Act IV Act V|
|Act II Quests||Act II Events |
|Quests Act III||Events Act III||Quests Act IV |