This mod/exactly map is using HL2 and HL2EP1/2 content. Story is short: you and other soldiers of starship troopers has been attacked in the night by many of Antlions, sometimes came Antlion Guards too. You can only defense the building and your life before group of Antlions.
Tip: To make a skybox, use the tools/toolsskybox texture then under map>Map Properties fill in the SkyBox Texture Name field with the name of the texture you want to use. Do not include anything like "rt" or "hdrup", only include the sky_day and the numbers that follow.