Exposed map, so I have one river crossing to defend. Used double walls, two rows thick and test you didn't leave a hole. The AI sort of stopped when it hit one wall. Towers, towers, towers to wear him down. Upgrade anything that helps range and towers. Eventually he runs out of gold and is all bow. Then use river to build Trireme and destroy his towers by river. Lose about three ships to bring down a tower, and need to watch that he isn't repairing them. The end game was cross with a couple cats, defend the shore with triremes. He surrendered, coward.
The Hard setting has twice tried to bum rush me in tool age. Comes with slew of ax and bow. Second time he stopped at walls and towers. Then came back in Bronze with cavalry. That died on walls. Finally he brought cats, but by then the boats on river could stop those. Sit back, harvest all your resources, then one transport to cross river. Leave the walls in place for tourist of a later era to stand on and take selfies.