The catapult becomes available as soon as you earn a handful of kills from any of Aoc Gold the shops in the zone. It's faster than the demolisher, but it also deals less damage, and is a 1-person vehicle. If your faction has the advantage and only needs support fire, the Catapult is the best option to get to the front line and start doing some damage to the enemy building. However, if you're still advancing and capturing checkpoints, it might be best to continue ramping up your kill count and get access to the Demolishers.