most of this about the gamemode will be copied from Dren's thread, but I'm posting it here beacause he seems to have lost interest
this gamemode is very fun in Unreal Tournament and I'm sure it be adorable in blockland too
I suggest reading this:
http://www.gamefaqs.com/pc/914986-unreal-tournament-2004/faqs/29337specifically, "The Concept of Onslaught and How to Win"
the rest is for actually playing the game
but...
if you're going to try out Warfare instead, which will certainly be more difficult, although much more fun, I suggest reading this
as well as the previous linkhttp://www.destructoid.com/hands-on-unreal-tournament-3-s-warfare--49084.phtmlthere would have to be a special brick for the cores, and one for the nodes as well
probably two bricks for both teams' cores, I'm not sure how that works
the hardest part would probably be the nodes, because they have to create links from one core, to each other, and eventually to the other core

The blue thing at the top is blue team's core
you can see that it is linking to 3 and 5. the red core is also linking to 4 and 6, because they are available for control of red team, like 3 and 5 are for blue team
1 and 2 (you can't see the number for 1, but it's the one to the bottom right of 2) are not controlled by either, which you can tell by the fact that the lines are white
the map is from the viewpoint of blue team
the white circles are the nodes that they can currently take over
the must take over one of those nodes in order to unlock 1 and 2, allowing them to be taken over
at this point there will probably be a lot of combat between them and red team over the nodes
after taking over 1
or 2 (but only taking over 2 will only unlock 4, and 1 for 6 the same way), the red core is now vulnerable, and the must destroy it
if you still need a better explanation, you should probably install the UT2004 demo and play it yourself