Here is some gameplay (from the alien camp point of view). This is another good example of gameplay.

Apparently both Natural Selection and Tremulous were inspired by the Gloom mod for Quake 2, according to the Tremulous FAQ:

Development on Tremulous began long before NS was in the public domain. If Tremulous is inspired by anything, it is inspired by Gloom for Quake 2. NS has a similar ancestry (please see Game Developer Magazine February 2001 issue); this is probably where the confusion arises.