Head to the Tyras camp and speak with General Hining. He will explain that the catapult is an essential part of the camp's defence and that without it, it could mean disaster for the camp. He will give you permission to help repair the catapult. He tells you that you need to find the missing engineer whom he suspects is off somewhere in the forest. He will also give you an Engineer's letter to bring to him. If you lose it, speak to General Hining again to receive another one.