I thought I'd mention the Medieval Total War 2 engine. It's a fairly easy tool to work with but it's horribly restricted. You'd want to use it to create a game about medieval warfare and not much else.
It's successor, the Empire Total War engine is more powerful and has less restrictions. It's also a bit harder to learn and until proper documentation have been done I think that Medieval 2 will be the primary engine to work with. Both deserves to be mentioned.