The Iron Challenge is a challenge for skilled players.
DescriptionIn order to unlock the Iron Challenge on a level, the player must have got all three Stars/points for the Campaign mode of that level. The player only has one life and has to withstand a long, gruelling wave of enemies. Certain towers are locked in this challenge, most often it is two towers, but some stages are limited to only one. Like the Heroic Challenge, upgrade paths are limited to a certain level. Completion of the challenge is rewarded with a Star. When completed, the flag of the respective area on the campaign mode map will gain a silver tint.
Unlike other modes, the Iron Challenge does not indicate the number of enemies which will attack in the wave from each path - the enemy number is replaced with two question marks. It could be because they don't really want the player to know how much enemies there are so that they won't let them know how long it takes the Iron Challenge to finish.
- When playing an Iron Challenge on a level that featured a boss, you will often see a result of the boss having been defeated. For example, the Juggernaut's head is lying on the side of the road. This doesn't occur for all boss levels.