It is a tricky technique called geo-merging, you can find all about how to do it here. Also, double posting is looked down upon here, hazza, next time remember to just edit your post if you need to add something when no one has posted after you.
Finally, someone else who makes maps more like Bungie. Very good. I dl'd and played it and it fits right in with the maps I have made.
I'm personally sick of "pros" and MLG stuff, but this map is really fun. I think it's absolutely crazy that that much time was spent on geomerging. geomerging makes me go insane, it takes so long. And anyway, how is it that each side of the map is perfectly symmetrical? mine usually is off by a little bit.
Very good map downloaded the other day, and i held a custom game, the time put into this map showed very clearly
This map just sets an even higher standard for merging and forging maps. It is just amazing to see the little details and how much was truly put into this