Bonus levels:
While you are at the level select screen press X to bring up "Lost Levels."
Then Beat the game with Gold ranks to open up more levels.
Praetorian Camp:
3 Gold ranks
The Forum Romanum:
5 Gold ranks
The Forum of Trajan:
7 Gold ranks
Addressing the Senate:
9 Gold ranks
Caesar's Sanctum:
12 Gold ranks
The Silk Road:
15 Gold ranks
The Gate of Supreme Harmony:
18 Gold ranks
The Steam Tower:
22 Gold ranks
The Geothermal Tunnels:
24 Gold ranks
The Corridors of Power:
26 Gold ranks
The Gallery:
31 Gold ranks