I went to Coral Castle as well. The tour guide mentioned that Mr. Leedskalnin had tuberculosis and left colder northern climates in the hope that warm Florida sun would cure his TB. Apparently it worked, and it went into remission.

Part of the mystery of the construction was that he did it all at night, and stopped if he caught neighbors watching. As a budding engineer, the construction methods of Coral Castle didn't look all that mysterious to me (although no less awesome--not doable by conventional means). I believe he used pulleys and giant a-frames to lift the blocks of stone he chiseled out. He could have used car motors or something to do the lifting, but with enough pulleys, he could have done it only by muscle power.

As to the hardness of the coral, I can't attest to that. A lot of it looked like coral in sandstone to me, which while still being quite hard, it can be chipped away slowly with enough persistence. At the location of the castle, coral rock is prevalent, and there are several large pits around the castle where he lifted the walls out of the ground.

What is amazing to me is the time it must have taken and the artistry of the results. He had chairs carved out of the stone walls (unpadded) that were more comfortable than some of the modern padded chairs sold today. He had a several ton block of stone for a door that was balanced on a pivot so that you could move it with a finger, but is so heavy that they had trouble reseating it with a modern crane after a hurricane threw it off.

Overall, I'd say it's a very beautiful, although lonely and haunted, place.