You might be a new player and recognize a few em placed ballistas around the map. This guide teaches you how to build one. You'll want to pick the engineer merc, or have the tool box and hammer with the smith perk. When you spawn as an engineer, you'll have 3 ammo to use with the tool box. All you gotta do is go to a ammo box, get 3 more (giving you a total of 6 ammo) and then equip the toolbox. With the tool box equipped, press R to cycle through what to build. You can build a small wall to block arrows, a row of spikes to slow attackers, and finally a ballista. You can only see the ballista as an option if you have 6 ammo in the toolbox . All you gotta do is place it in a good spot(it's kinda finicky) and then you're almost done. Just wait for it to build by itself or wack it with your hammer to speed it's construction. When it's done building, you're all set! Just interact with it and you'll take control of it. Don't shoot teammates and aim for Calvary in particular. To refill it's ammo, dismount it and wack it a dozen or so time to reload it's bolt supply. I hope this helps.