In Call of Duty: Modern Warfare 3 for the PC, PS3, Wii, and Xbox 360, levels in Survival Mode are unlocked gradually as you progress. The levels you start out with are fairly easy, and of course get more difficult as you go along.
Here are the levels that can be unlocked, and what level you need to reach in order to unlock them.
Tier 1 Levels – Easy difficulty, unlocked from the start
Interchange
Resistance
Underground
Village
Tier 2 Levels – Regular difficulty, unlocked at level 8
Carbon
Dome
Mission
Seatown
Tier 3 Levels – Hard difficulty, unlocked at level 15
Bootleg
Fallen
Hardhat
Outpost
Tier 4 Levels – Insane difficulty, unlocked at level 25
Arkaden
Bakaara
Downturn
Lockdown
