Resident has released a nifty little gametype for UT2004 called Defense Invasion, which modifies the regular Invasion gametype in a small but critical way. Here's more:
You can download the file here.This is a little modification of the Invasion mode. In this Gametype, the Monsters respawn in different locations than the Playerstarts. The Monsters will respawn in one of the edges of the map and your mission is to prevent them for advancing further in the map, in a "Defend this position" style of maps.