Possible Clues:

• Crescent moon's form
• Ordered toilet mesh that lets in light
• Aperture in instrument plugged with open fabric
• Instrument protects catch over the door for light
• Cement sealing clear opening in dome
• Plucky type hides bag in alcove
• Crescent-shaped object
• Fortification
• Half-moon-shaped window
• Instrument used to secure trap in fortification