It may not be apparent that Serious Sam: Siberian Mayhem is a standalone expansion to Serious Sam 4, mainly because it doesn’t have four in the title, but it is. Inserting itself into the main game as a lost episode opens the door for creativity without fitting into a mold. Timelock Studio (the developer formed out of making this expansion) offers up a challenging and memorable game, a rare case where it exceeds its predecessor. Croteam put hundreds of enemies onscreen and worked it around the gameplay. Timelock Studio instead built gameplay around the hundreds of enemies onscreen, and that’s the key to its brilliance and brutal difficulty within it.