Siegeworld started off as a way to play mega-battles set in the 40K universe. The problem is that the term "mega-battle" means many different things for different players, and what Siegeworld really is, is generally not any of these things.
When some people think of a mega battle, they think of what I call a titan battle, lots of players show up with forces made up mostly of titan and superheavy tank models (and maybe some regular tanks too) and duke it out on a long table. While these can be huge fun, they do not really represent a large battle as such, instead they represent a battle between large war engines. In many respects a titan battle is like playing the old Games Workshop "Adeptus Titanicus" game where complete armies of warlord titans fought it out with each other and infantry and tanks were virtually non-existent.
The other type of mega-battle you commonly see is simply a lot of players playing standard armies on either a long table or lots of related small tables and they are allowed to have war machines. These, in my mind, are not really mega-battles between massive forces, they are interconnected battles between a large number of small forces.
Siegeworld isn't either of these types of "mega-battle". Siegeworld uses an enormous battlefield (in the thousands or tens of thousands of square feet) whcih is the floor of a large conference room or similar facility. Further, players can bring any army of any size (we regularly have armies from 2,000 to 40,000 points) with any composition. The lowliest trooper to the most massive war machine have a place. This makes Siegeworld a single battle where every thing gets a chance to fight.
The first Siegeworld "Siege" occurred in 2003. Since that was pre-apocalypse, rules additions were designed to allow and encourage the flow of units around the battlefield so that armies acted in a more concentrated fashion. When a section of defense starts to crumble, commanders (if they have the resources) can bring a huge weight to bear to try and crack through, or hold back the oncoming tide. Further, rules for cutsom units were created. These are still posted in the Rules Archive section, but are long since obsolete.
Since the advent of Apocalypse, Siegeworld has focused on all aspects of playing this variety of 40K game. in 2009 we are moving our main event from January to June, and moving to a significantly large facility. We also now host regular "mini-sieges" which are still Apocalypse games in the 40,000 point range. We hope, over time, to become the premier destination for those who see 3000 points as just a good starting point.