Challenge maps are part of the single-player and co-op modes for Iron Harvest. The collection of maps is for experienced commanders, with specific tasks to solve. Each map has its own "hook" and the goal is to master each one of them.
For example: One map might be a sort of "horde mode" map in which the goal is to defend an area as long as possible. The longer the area is defended and the more enemies that are killed, the more points that will be earn.
A couple of challenge maps especially designed for co-op. They run along the lines of "coordinated attacks on enemy bases" or "stay alive until your co-op partner reaches you."