When the plate was clear he took the advice literally and transformed into the hulking dragon. In some ways being captive in his own dungeons had some positives. One being that their dungeons were so huge that in dragon form he still did not touch the ceiling. The negative however was that he knew there was no escaping on his own. He was trapped there with only Shadow for company until either death eaters let him out, someone came to rescue him, or he perished a most miserable death.