So, the top two Defensive Civs are Babylon (Strongest Walls, cheapest Towers), and Greeks (Strongest Towers, 3rd strongest Walls, powerful range).
Romans and Egyptian are both solid - Romans have their unique tech, Arch Construction which gives a solid amount of Crush armor, making their buildings much more tanky against Siege compared to others, as well as making them cheaper on the stone. I believe Romans also have access to an advisor that grants their 3rd rank of Towers and Walls? Not sure there.
Egyptians have an additional Wall upgrade making their walls the 2nd strongest.
Persians lack a 2nd Tower upgrade, making their towers quite a bit weaker, and Celts lack any 2nd upgrade at all without a specific Advisor, meaning you have to dedicate quite a bit to Celt's defensive capabilities.