These massive crustaceans build a shell out of rock fragments (like caddisfly larvae). They vary in size from about a foot in diameter to the size of a large building. The collect up the gravel that tends to accumulate between layers, eating the lichens and plant life off of stray rock fragments, then adding them to their shell. When they need to grow, the crabs molt, and leave behind their rocky shell which forms a home and shelter for a number of other creatures such as Grappler Ants. Occasionally a tool or weapon will be found, and incorporated into the shell in the same way. The action of the fortress crabs counteracts the tendency of rocks to break into smaller fragments due to collisions, extreme weather, animals, etc.