The Alien_base.zip file you have requested: Maps Levels Missions / Deathmatch / Alien Base - 1.17mb is not available. Perhaps try browsing all files for the game Serious Sam: The Second Encounter and upload any files you want to share or are missing. This is the only information we could locate:
A small deathmatch map for Serious Sam: The Second Encounter. Best played with two - four players.