Siege Engines
Originally castles were built of wood. Since they were wooden the opposing army could easily burn it down. The first siege weapons were called Greek fire. Greek fire was a mixture of gunpowder and oil that when you lit it would explode showering every thing in fire. So they started to build castles of stone making them the best defense any king could have. So the opposing army came up with more advance and stronger weapons to destroy the castles walls inventing siege engines.
