The conquest war elephant is possibly the BEST knight replacement. I made an experiment with the combat calculator:
The defender is a fortified musketman in a city on a grassland tile:
A veteran Knight has a 20% chance to win. About (1/5)
An elite Knight has a 30% chance to win. About (1/3)
A veteran war elephant has a 30% chance to win. About (1/3)
An elite war elephant has a 40% chance to win. About (2/5)
An Elite War Elephant has a bigger chance of winning this battle than a veteran cavalry!
It doesn't require ANY resources at all. I think this undoubtly is the MOST underestimated unit in the game.
The defender is a fortified musketman in a city on a grassland tile:
A veteran Knight has a 20% chance to win. About (1/5)
An elite Knight has a 30% chance to win. About (1/3)
A veteran war elephant has a 30% chance to win. About (1/3)
An elite war elephant has a 40% chance to win. About (2/5)
An Elite War Elephant has a bigger chance of winning this battle than a veteran cavalry!
It doesn't require ANY resources at all. I think this undoubtly is the MOST underestimated unit in the game.