On Mundus Magnus, many of the rebel cities are heavily defended. But the armies have a trait that makes them immobile (unable to leave the city and go conquering everywhere).

One of the especially strong ones that comes to mind is Syracuse:

8 Hoplites (2 silver chevron, silver defence and weapon upgrades).
2 Cav (2 silver chevron, silver defence and weapon upgrades)
General (decent upgrades too).

It's quite difficult to take with your starting armies because of the upgraded troops.