This morning I had cause to do a quick Wikipedia lookup on Samurai. Which, inevitably, led me to an article about Korean Turtle ships, the first ironclads – dating from, would you believe, the early 15th. Century. The article suffers from some dodgy editing, but it all becomes a bit more clear if you also read up on the contemporaneous Panokseon vessels.
Fascinating period of history. Particularly when it turns out that there’s considerable controversy over the design of the ships, and whether they were truly ironclads at all.