Great idea! Like silver weapons could do more damage against werewolves, holy weapons against undead, and wooden(?) weapons against vampires.I like the idea of a day/night cycle. I also liked the idea of nocturnal mobs. Imagine if the castle had problems with vampires, wearwolves, or undead when the sun went down? Certain bosses could only spawn if there was no sunlight, then the introduction of "light" or 'holy" weapons could be used to damage them, etc... Lots of possibilities.
They would emerge during the night and kill unwary adventurers.