To make it clear: It won't work in 2.3 or its test releases. You need Nexuiz from trunk.
Nexuiz SVN trunk (not branch-2.0) has an Assault game mode similiar to what Unreal Tournament has.
Here's a demo map: http://savage747.sa.funpic.de/assault2.pk3
Start with: nexuiz +set g_assault 1 +map assault2
Defenders have to... defend stuff (in this case buttons that the attackers must push):

Attackers have to push buttons or destroy walls so they can access other areas:


This gamemode needs special maps. I suck at mapping (see the demo map, which is sorta playable... but it just looks horrid).
By default every round takes 5 minutes (this can be changed by the map).
Have fun and start mapping!