This version featured a new level, which replaced the secret levels.

Sometimes, a whole level may be designed as a secret level.

Each system consists of 4 regular levels and 1 secret level.

However the player may not save nor load a game on the secret level.

It consists of eight regular levels and one secret level.

Though not a secret level, there is an Easter egg in episode two.

Descent features 30 levels, three of which are secret levels.

It might be secret level, but certainly not anything higher than that.

She might still be involved in running things, on some deep, very secret level.

Similarly, the secret level can't be accessed if said conditions aren't met.