Even though in history "laying siege" means surrounding a town or city and waiting until they no longer have the will to resist and in the SCA battles only last an hour or two so this style of siege is not possible, siege weapons are still very much alive. Siege engineering is the art of designing machines to destroy castle walls from afar. To that end siege engines deliver 100-pound rocks which could have destroyed castle wall or ballista javelins which could have killed several armoured knights in a single shot.
For safety reasons we don't use historical ammunition, instead we use safe objects to represent them. For example a 6 inch foam ball represents the 100-pound rock if it hits a wall that wall is considered destroyed, or a cluster of four tennis balls represents a 10-pound rock which would have been used against people, or the ballista javelin is replaced by a tennis ball tipped pipe.
The engines themselves are built and powered just like their historical ancestors. We build counter weight and man powered Trebuchets, torsion powered Ballista and Catapults. The deadlier, the better.
Siege Weapons are a great way for those who want to be involved with the fighting and be on the combat field, but don't want to be in the thick of combat. As one engine needs several people to operate, even if you can't build an engine of your own, you can still help deliver love from afar as part of the crew for an engine.
Baron Jonathas has built several engines and taught classes on Siege. For more information feel free to contact him at Jonathas AT RedFoxDen DOT org.