Can't finish off enemy (2 hour game against Moderate AI :c )

I'm an incredible noob, and going against a Moderate AI (Greek vs. Norse) I LITERALLY could not end the game. With thousands of resources, and a near infinite stream of me sending units and seige their way, the enemy just would keep on rebuilding things as I destroyed them and I just can't kill them.

After 2 hours of sending wave after wave against them I resigned because it's not worth my time and it was never going to happen. With essentially infinite resources and population I was unable to kill them. Wth am I doing wrong?