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