From The TimeSplitters Wiki

Chemical Plant
Size	Large
Location	A Chemical Plant (abandoned?)
Climate	None
Unlockable	Beating the first 3 story levels
Recommended Bots	N/A
Recommended Weapons	N/A

Chemical Plant has a main path way at the ground which is surrounded by multi-story building in which you can travel in. There are also a few ledges for which to snipe from.