Road as grin (simile)
"They reached a paved road that crossed their path like a grin"
The road curved around in front of them in an elongated U-shape, reminding the narrator of a smile or a grin.
House as castle (simile)
"The one to the right was a Tudor that commanded its small rise like a castle".
One of the houses that Teddy could see on Shutter Island was at the top of a small hill and seemed to be the focal point of the island, which made it seem as though it had been constructed for royalty or to have the best view of potential invaders.
Eyes being prodded (simile)
"The eyes themselves were too wide, as if something hot were prodding them from inside her head".
The photograph of Rachel makes it clear that she is tormented by things that most likely nobody but her could see. Her eyes were wider than normal, as if she was incredibly surprised or shocked by something, prompting the narrator to liken their expression to someone being prodded with something hot.
Air like trapped breath (simile)
"Outside, the air felt like trapped breath."
The air before the hurricane was thick and not at all clear, much like breath that's been held too long before being released. The simile also pertains to the feeling that the island was holding its breath waiting for the inevitable arrival of the storm.
Roof trim like a pompadour (simile)
"Ward A had lost four windows, and several sections of flashing were curled back like pompadours on the roof."
A pompadour is a hairstyle with a quiff that bends back in a high peak which is what the flashing, or trim, looked like when it had been blown back off the roof.
