The SCA rules governing siege engines can be found at www.sca.org/officers/marshal/combat/siege
The Midrealm Rules governing siege engines can be found at http://news.midrealm.info/Marshal/index.php?option=com_docman&task=cat_view&gid=47&Itemid=26
When siege engines are used in SCA combat, they are also governed by the rules for armored combat which can be found at http://www.sca.org/officers/marshal/combat/armored/index.html
When siege engines are used on the archery range, they are governed by the archery rules. These can be found at www.midrealm.org/marshal/archery/ The archery range may be the only place at an event with enough room to fire a siege engine. They may shoot there for demonstration purposes. These may include engines which were not made for SCA combat.
DocMo Kincain came up with the design used by most of the ballistae in the SCA. This is the giant crossbow design which technically is called a 15th century arbalest. DocMo sells design manuals, parts, and complete arbalests. Picture courtesy of DocMo. His website is at www.ballistaplans.com
Master Jean Paul Pierrepont of the Dark Horde also sells supplies for siege engines, particularly for ammo. His site is at www.thesiegeshop.com At his site you can find the instructions for making pool noodle tips for combat ballista bolts.
The Midrealm is not the only kingdom with siege engines. The East Kingdom also has quite a crew. Their website can be found at siege.eastkingdom.org
The Interkingdom Siege Weapon Challenge (ISWC) is a system for scoring a series of shots with a consistent set of standards. This system is for all the kingdoms in the SCA. It is similar to the IKAC used in archery. The ISWC scores are recorded by Load Caedmon Wilson. His site contains the rules and the scores for the ISWC. It can be found at mysite.verizon.net/interkingdom/iswc/main.html
This is my household, House Ironwolf. Their website can be found at www.ironwolf.org
Shadowclans built the red ballistae seen in several pictures on this website. DocMo is a member of this household. Their website is at www.shadowclans.org/
Captain Dyderich Wolfhart also makes siege engines in the Midrealm. His instructions can be found at www.geocities.com/dyderich/siege.html
The Caidan Royal Artillery Corps has a website at www.batteredtower.com
Additional information on building and using siege engines can be found at the following non-SCA sites:
www.thinkgeek.com/geektoys/warfare/81e6/ for model siege engine kits.
If you look very carefully, you should be able to see eight ballista bolts in the air over the fort. Picture from pennsic.net.