I think Agartha is pretty much the classic turtling nation to be honest, at least EA and MA. You'll struggle against an early rush, but if you can keep the wolves from the door (diplomatically, militarily or by any means necessary
) they can become strong in the mid to late game.
One advantage you can use with Agartha (not sure if anyone else has tried it) is to go amphibious. If you're in a game without any other water nations it's an easy way to mitigate against an early rush. By the time the other nations can get underwater in any reasonable force, you should be ready to come out guns blazing so to speak.