Atlantis was originally a mountainous island, with a low hilly mountain in the middle and mountains in the north. But after marrying Cleito, Poseidon altered Atlantis' geography, as described in Critias:
"He broke it off all round about; and he made circular belts of sea and land closing one another alternately, some greater, some smaller, two being of land and three of sea, which he carved as it were out of the midst of the island; and these belts were it even distances on all sides."
In the middle of these rings was the center of the original island. Poseidon then made two springs on the central island, one each of cold water and hot water. Out beyond the third water ring was a plain that covered most of the island.
Atlantis was approximately 345 miles long and 230 miles wide. Starting from the outside, the first water ring and land ring were each 1,800 feet wide. The next water ring and land ring were each 1,200 feet wide. The innermost water ring was 600 feet wide, and the central island was a half mile across.
A stone wall plated with metal was built on each land ring and on the central island. The stone was red, white and black. The metals used for the walls were orichalcum for the wall surrounding the city, tin for the wall on the middle ring and brass for the outer ring's wall. There was also an outer stone wall that circled around the outermost ring.
The city of Atlantis was on the center island. In the center of the city was a temple to Cleito and Poseidon that was blocked off by an enclosure made of gold. Outside the enclosure was a temple built for Poseidon, made with silver, ivory, orichalcum and gold.
There was also a palace, "which they continued to ornament in successive generations, every king surpassing the one who went before him," according to Critias.
To be able to sail to and from the central island, the people of Atlantis dug a canal through the outer island to the central island wide enough for large ships to pass through. The canal was 300 feet wide and 100 feet deep.
The people on Atlantis connected bridges to the land rings from the city. Next to the bridges, covered channels were built for ships to pass through. Guard towers were placed next to the channels, made with the same type of stone as the walls.
The plain on the island outside the rings was used for agriculture. There were vineyards, orchards and cornfields, among other types of agrarian cultivation.
Open-air and covered reservoirs were built around both the hot and cold springs. The covered reservoirs were used for bathhouses, which were built over them.
No one knows what kind of metal orichalcum was. Some believe it was copper, or some kind of combination metal including copper. The name "orichalcum" comes from the Greek words "mountain" and "copper."