Cinder Blocks are used in clan warfare. Each block costs 150 meat.
You can build a wall out of Cinder Blocks, to make it more difficult for enemy ground forces to invade your Clan Hall and steal your Goodies. This wall can contain no more than 1000 blocks (which should be enough, anyway).
Cinder Blocks can be destroyed by an opposing clan's forces, forcing your clan to purchase more to maintain a complete wall.
But be careful - Grrrrrlgoyles do not get blocked by Cinder Blocks, they only work against the Knob Goblin Teenage Grunts!