The Metro: Last Light - Tower Pack is now available on Steam, offering more content for Metro: Last Light, 4A Games' first-person shooter, which is on sale in celebration. According to the announcement:

Fight your way up through a heavily guarded tower stronghold. Each level is increasingly difficult- Can you make it to the top? Compare your time and other stats against your friends with the added leaderboard functionality and try again.

Note: This purchasable item is included in the Metro: Last Light Season Pass.