Your looking forward through the firebox door of Caerphilly Castle into the interior of her firebox. The rusted bolt-like objects on the roof, or crownsheet, are the crownsheet stays. Below those at the front you can see the opening to the flues and larger superheater flues. Casting a shadow over the flues is the brick arch. The brick arch directs heat from the fire up and over then into the flues creating a draft against the firebox door each time it is opened resulting in smoother combustion and heat generation. (See the article What a Locomotive Brick Arch Does in the Firebox.)