Similar thing happened to me in my game using Huayna Capac (Emporer level). 5 axes "randomly" arrived outside the capital, and I only had Quechuas to defend it... game over. If I had underestimated my rivals, and one of them turned up with a stack of doom outside my capital, I could live with it, and learn from it. To lose the game due to some arbitrary slice of bad fortune was just lame.
I also think the "Health Plant" event is inversely over powered. +2 health for all cities near the start of the game is ludicrously helpful at the higher levels.
IMHO any random event (good or bad) that's so powerful it can literally decide the fate of the campaign, has no place in a "strategy game" like civ.