It's two-dimensional with the illusion of three, regardless of aiming or other such appearances. When you're walking up the steps in Harrogoth, you're actually just walking on a plain surface that is drawn to look like steps. This is why the perspective option in the video menu was made, though each tile is separated from the others (trees appear to sway when walking away from them with 3DP active).
It can often be hard to determine, AFAIK many less advanced first person shooters are actually made up out of only 2d graphics but in such a way it appears to be 3d (maybe not as common nowadys, but still)
DII is most definitely 2D. Anyone ever played Mario RPG: Legend of the Seven Stars on the SNES? It's isometric 2D like Diablo. That even has elevation changes though lol. I like how effect they were able to make the illusion of elevation by putting in stairs at the right angle and blocking of the "ledges."