A ballista is a gigantic crossbow. A weapon of war, it was used to launch large darts or stone projectiles. Torsion supplied the necessary force. The ballista was first developed by the ancient Greeks and later refined by the Romans. It was a standard piece of equipment among the Roman legions. Highly mobile, but difficult to make, it was largely displaced by the catapult during the Middle Ages. A simple form of ballista would resemble a mounted bow.
Begin with the base. The frame can be a small table, bench or cart. If you want to move it around, use something with wheels. Take two 2-by-4s of equal height and mount them vertically on the top of your base. You should place one at each end of the base. Fasten them securely and place a third 2-by-4 horizontally across the top of the other two. Secure it carefully.
Take a thin wooden beam and bend it into the shape of a bow. When bent, the beam should be as long as the space in between the two uprights on your base. To bend the beam, use steam or soak it in water for several days. Gradually work it into the proper shape. This process may take several days.
Fix each end of the bow to the inside of one of the uprights. The bow should be fixed halfway up the height of the two uprights and at an angle of about 45 degrees. The front of the bow should be pointing slightly upward.
Use three very long 2-by-4s to make something like a chute. Lean a 2-by-4 up against the side of the base behind the bow. The 2-by-4 should be long enough to reach from the ground to a few feet beyond the front of the bow. Make sure the other 2-by-4s are the same length. Lay the first 2-by-4 flat on the ground or on a table. Stand up each of the other 2-by-4s on either side of the central beam. Join them together. You should have a chute. Replace the chute against the edge of the base. You can support it from behind with a metal brace or wooden wedge.
Attach a winch inside the chute and not too far from the ground. You can use a wooden spool that will fit inside the chute. Just mount it so it will turn freely above the floor of the chute. Add handles to either side. Tie a heavy rope across the bow. Tie another rope to the winch. Tie the other end of this rope to a hook. Place the hook on the middle of the bowstring and crank the winch. Let go of the winch to launch your projectiles.
Try attaching a small bucket or cup to the inside of the bowstring. You can use this to hold your projectiles prior to launching.
Do not aim the ballista at any people or breakable objects. Even a small ballista or bow can be very dangerous.