The Chapter 9 Secret Key can be found pretty quickly in DOOM The Dark Ages, but you'll have to know where to go in this somewhat confusing level.

The Chapter 12 Secret Key location in DOOM The Dark Ages is not immediately obvious, hidden in an elevator shaft.

To find the Secret Key in Chapter 10, DOOM The Dark Ages players should continue through the mission as normal, following mission markers, until you get through the gate barring your entry forth.