Ballista

Siege Monster.  The ballista deals double damage to objects and structures.

Crew. The ballista starts combat with a crew of three, this crew is to represented by humanoid creatures (default to using the Bandit stat block). When a crew member is lost, all actions for the ballista gain the recharge trait at 3. For each additional crew member lost the recharge goes up by two. If all crew members are killed, the ballista is destroyed. The ballista may take more than three crew, up to six, but this confers no benefit.

Reliability. The ballista, when damaged will start to lose efficacy, if the ballista has taken half or more of its starting hit points in damage, it has disadvantage on all attack rolls, and all creatures targeted by a ballista's attack gain advantage on saving throws against its attacks. 

Nonliving. The ballista does receive any benefit from healing from spells or abilities. Instead someone can spend eight or more hours, and make a DC 15 Intelligence (Smith's Tools) tool check to repair all damage to the ballista.

Solid Shot. Ranged Weapon Attack: +6 to hit, range 700/2,100 ft., one target. Hit: 33 (6d10) piercing damage.

Hot Shot. Ranged Weapon Attack: +6 to hit, range 700/2,100 ft., one target. Hit: 22 (4d10) piercing plus 7 (2d6) fire damage. Creatures within 5 ft. of the target must make a DC 13 Dexterity saving throw or take 7 (2d6) fire damage.

Grapnel Shot. Ranged Weapon Attack: +6 to hit, range 500/1,500 ft., one target. Hit: 11 (2d10) piercing damage. If the target is a creature, it must make a DC 14 Strength saving throw or be restrained, it can make a DC 14 strength check to break free of this restraint at the start of each of its turns. If the target is a structure, it can now be scaled as if the outer wall were difficult terrain.

Take Crew. If any willing humanoid creatures are within 5 ft. of the ballista, they can be taken into the crew, such a creature cannot take actions, or bonus actions and uses the ballista's initiative, saving throws, armor class, and hit points. Creatures that are part of the crew cannot be targeted by spells, attacks, or abilities unless such effects target an area of effect. 

Detach Crew. The ballista can detach any number of crew to be free to act independently, if all crew are detached, the ballista cannot take actions and is considered destroyed if no crew rejoin within three rounds. 

Sacrifice Crew. The ballista's crew can sacrifice themselves to prevent damage to the ballista, if this occurs, up to two crew members take damage instead of the ballista.