Ancient Diablo stuck behind walls & bars?

[webwalker]

For the first time in 10 years, I went into the Dungeon to get the black key from Ancient Diablo.
After the long, tedious grind to get to Ancient UberFallenAngel (it's not Diablo really), I found there was no place on any wall that let him out to me, nor let me in to him.

Finally I fought him through the bars and grabbed the loot with Telekinesis.

Is there a map of this layer showing what part of the wall can be opened/slid/destroyed to get toe-to-toe with AncientDiablo?

[Hoerby]

If you go a bit back, from  where you killed Diablo, there is at the right a small way to a firedoor.
If you go through you will find the seven and the way through to the Dia-clone

[webwalker]

That is what I thought and that is what I attempted.  The "Mephy Portal" is behind bars, I can't get through.  That really skinny passage was a dead end to me.  Did I not try the right thing?