Try re-exploring some of the levels whose paintings are located near the metal doors
it's funny, after i turned off my computer i thought the solution would be.. just what you said! i appreciate the hint.
i wanted to wait until i was finished to say... this is by far my favourite doom mod i've ever played! it's so fun and imaginative. i love love love the regular evil doom so much, but sometimes it's great to go outside of that. i cannot thank you enough for this incredible work!!! easy 10/10 for me. i'm tempted to play 2 and 3 right away but i really want to savour them and spread them out.