A Tiny Blue Citadel

A flash! Something metallic glinted by the baseboard! Aah! I threw myself backward into my wooden barricade, shivering right down to my plush hem! But wait. Utter stillness. Peering out from between two upright hardcover books, I noticed a discarded plastic bottle cap resting upside down upon the parquet floor. It possesses ridged walls and a flawless circular circumference! An exquisite subterranean chamber for visiting specimens! Moving with tactical precision, I nudged the cap across the floor with my royal robes, flipping it neatly beside the clothes brush moat. A magnificent outpost.