Monster's CR. Depending on what the DM comes up with the CR is up for interpretation. Use your judgement of what the actual CR is, this is a baseline for ruls and you to customize at your leisure.

Siege Machine Operators. This is an inanimate object therefore must be operated by men or women who know how to use it. There must be at least 3 Siege Machine Operators for this siege machine to be used. There are normally 3-5 Siege Machine Operators that work the Catapult. The NPCs listed below are some examples of who the DM can use as Siege Machine Operators.

• Novices: Guard, in the Basic Rules
• Skilled: Soldier, in the Guildmasters' Guide To Ravnica
• Expert: Veteran, in the Basic Rules

 Siege Machine Operators. There are three skill levels of the Siege Machine Operators 

• Novices. Novices are unskilled users of this siege machine. When the Fire action is taken roll a percentile die. If rolled 50% or lower, add a -10 to the To Hit modifier for that Fire action.
• Skilled. Skilled users have been in regular use of the siege machine. When the Fire action is taken roll a percentile die. If rolled 25% or lower, add a -5 to the To Hit modifier for that Fire action.
• Experts. Expert users of this siege machine know it's every nook and cranny. The experts have used the Catapult so much that they are almost 100 percent accurate with it.

Ammunition. There are many types of ammunition to be used with a catapult, it is up to the DM what type of ammunition is used. Most commonly used is boulders but anyone who uses it can get creative. Some examples are listed below.

• Boulder. 3d12 Bludgeoning Damage; The basic and most effective ammunition used to knock down walls when attacking settlements.
• Fire Stone. 1d12 Bludgeoning Damage,  2d12 Fire Damage; A large stone not as large as a boulder but covered in hay woven around the stone itself to create a ball. It is then lit on fire with a combustible liquid. Effective when used to burn down wooden buildings.
• Skeletal Swarm. 2d8+10 Slashing Damage; A skeletal swarm alternates its appearance between partially formed humanoid shapes and a chaotic, swirling mass. This type of ammunition is used by undead armies to quickly sew chaos on the battlefield. After this ammunition lands on the target there is a Skeletal Swarm at the point of impact. Skeletal Swarm is in the Ghosts of the Saltmarsh.

Make of the Catapult. The make of the catapult determines how durable it is to receiving damage. This is up to the DM's discretion of what the Catapult is made of. These are some examples that can be used.

• Cobbled Make. AP 16, Full HP: 24 (2d12) A poorly made Catapult, commonly made with spare wood, unfinished logs and common rope.
• Fine Make. AP 18, Full HP: 48 (4d12) A well made Catapult made with finely manufactured wood and treated ropes to make the Catapult durable to both the elements and to enemies.
• Metal Make. AP: 20, Full HP: 96 (8d12) A Catapult of this make is rarely made. It is incredibly resilient and made of pure metal. Even the ropes are made of full metal.

Inanimate Object.  Has Disadvantage to Dexterity Saving throws since it cannot react to anything. If there are any other rules regarding certain spells being cast on the Catapult it is up to the DM to decide the correct ruling.

Fire. When loaded the Siege Machine Operators can fire the catapult. After firing the Catapult one of the Siege Machine Operators must take the 

Reload. After the Catapult has taken the Fire action the Siege Machine Operators must take the Reload action to be able to Fire again.

Action, Fire Action Attack. Ranged Weapon Attack: +10 to hit, range against structures 500/1000 ft., against non-structures on the ground 150/500 , Hit:  Example Ammunition: Boulder (3d12 + 10) Bludgeoning damage.