I purchased DOOM 3 BFG edition, which includes The Ultimate DOOM, DOOM2, and DOOM3.
I've played the first two episodes (Knee-Deep In The Dead and The Shores of Hell) already, and I'm starting out on the third episode - Inferno.
Like each of the DOOM episodes, there are nine levels - eight normal levels and one secret level in each - I haven't had any success in finding the secret levels in any episode yet, although I managed to clear the first episode with 100% of all secrets and I'll be replaying the second episode soon to get 100% on this also, with any luck.
The nine levels in 'Inferno' are;
- E3 M1: Hell Keep
- E3 M2: Slough of Despair
- E3 M3: Pandemonium
- E3 M4: House of Pain
- E3 M5: Unholy Cathedral
- E3 M6: Mt. Erebus
- E3 M7: Limbo
- E3 M8: Dis
- E3 M9: ??? (Secret Level)
Where are all of the secrets in The Ultimate DOOM Episode 3 - 'Inferno'?