Would like to see the stats side by side, for your era, to see why you say the Dark Doorway "not be as opulent" but still consider better to build it instead of building the Black Tower. Or it is due to the size is easier to insert at your city?
Pretty much what xivarmy said
Because at 3FP's for a 3x3 the dark doorway is well worth building, but the same 3FP's for a 4x5 simply isn't as I have no need for all the other resources the dark tower provides.
I do have the dark tower built in most of my other cities, it just isn't efficient enough for my main one.
By the way Darth, you're on my friend list in my main city, I just play under a very slightly different name there. Just change 5 letters from my name here. I'm sure you'll be able to work it out ;-)