Steel Door?

[Necrosope]

I am at the part where there is a steel door that you must open. I threw the switch to "wake up" the zombies, and killed them, then made my way through the dungeon and killed everything that got in my way. I found the machine and turned it on, then went back to where the steel door was, killed a couple more zombies, then went around and killed a bunch more, but the door will not open.

I tried all kinds of stuff, but it just won't open. What did I miss?

Thanks for your help.

NS