Two Feet Thick Steel
Two feet thick steel was the alleged thickness and material of walls that were used to imprison the gargoyles.
On two separate occasions, the trio, once with Goliath, were trapped behind steel walls that popped up. Brooklyn clawed at the walls and arbitrarily declared the walls of steel to be "two feet thick!" None of the other gargoyles challenged Brooklyn on this statement. ("Generations", "To Serve Mankind")
It should be noted that in the canon episode "Leader of the Pack", when Brooklyn, Lexington and Bronx were imprisoned below deck on the Otoshiana Maru, Brooklyn determined that the walls were made of steel, but did not know how many feet thick.