that stopped him a moment ; he looked at laurence , and the corner of his mouth twisted upwards a little ; then he said , `` follow me , '' and led them , their hands still full of pistols and swords , around the edges of the meandering pond : clinging to the side of one grassy dune was a great domed structure built of long narrow mud bricks , the same pale straw color as the yellowed grass , with a single arched opening looking in , and a small window in the opposite wall which presently let in a shaft of sunlight to play upon the dark and shining pool of water that filled the interior .