All cardboard boxes

The Cobra Fortress is Cobra Commander's newest and latest creation. It can withstand a direct blast from a howitzer and not be destroyed.

In a diostory I am working on, Cobra Commander and Dr. Mindbender are creating some new types of super soldier when something goes wrong and unleashes nothing but zombies.

Needless to say in the end, the fortress gets overran by the zombies and Cobra Commander becomes one of them.

There will be more bits of the fortress coming very soon. Here is just part of the fortress.