Historians discovered that some form of Rocket existed. However, I doubt there being such thing as a crossbow machine gun. The possibility of Anti Air existing in medieval times is highly likely, especially because humans where extremely superstitious and believed anything they heard, so chances are, citizens where worried about being attacked by dragons.
An example is the Congreve rocket, at first, it was simply a pole, and some form of bomb at the top. Now, bombs might not have existed, but gunpowder did, and this is explosive.
Also I think that Rail Gun is going to modern. The game is still based on medieval times, and the anti air was just added in to stop the Dragons Minions etc, as these would have been popular beliefs in the medieval period.

