So just what was Belial's plan there?

[Ruvanal]

I wouldn't be surprised if they used the D2 engine heavily. Some of the things that make me think this is the various stairs/ramps in the game. I've been melee'd while standing at one end of a stairs/ramp and the mob has been at the other end. I've also seen things like vault ignore the travel distance up/down the stairs/ramp (ie, you still go the same distance on the screen even though you shouldn't if travelling up or down a stairs/ramp). It seems like only the spell effects are truly 3D (like they were in D2) as distance is not obeying 3D when dealing with stairs/ramps (this was highly prevalent in the Arcane Sanctuary in Act 2 of D2).

From what I have seen in the D2 game and the way looks in D3, the base form of the games is that of a RTS game. But you are limited to only one unit, your character. And locked into only looking at the world centered on that single unit. The base engine is probably newer than what was in D2, but with that type of functionality they are both going to look very similar to each other no matter what you do to dress them up. For those that do not know, a RTS game field is just a 2D grid with sections of it non-passable. The graphic art overlay for the non-passable areas and some of the connecting strips can give a visual illusion that you do things like go up and down in elevation if they are drawn correctly. Kudos to the art team if they pull this off well, not the programers or developers.

There have been some 3D RTS games in the past, in fact Total Annihilation was using 3D when it was released in 1997. So, you can definitely make a tile set that does use 3D effectively. Yes, it's more difficult to program for, but the realism it creates (effects of slopes, height, etc) can make a better game (as seen by the fact that there are still people playing TA to this day and they have a group working on even more refinement by rebuilding and improving on the original engine). Given this, Blizzard could have, and should have, done this with D3 especially with so many games now being so ubquitious with 3D (where in 1997 this wasn't so much the case).

I suggest that you read the 'Physics' section in what you linked. It is a 2D game with tricks to make it look like 3d effects in it. I looked at several game plays and to me it only came off as some tricks with tile/unit elevation parameters driving the art work display to make a pseudo 3D effect on some cases. The game showed no sign of real 3D game work that would allow such things as multiple objects using the same x/y coordinates with different z coordinates for effect height stacking or the effect of variable distance perspective due to objects being closer or farther away. I looked at several game play videos and to me it was all just a lot of 2D stuff tying to give an illusion of a 3D game world. When I can see a RTS that can easily have the effect of a steep cliff where there is a one micro tile x/y difference and a very large z height difference and you are still allowed to send a unit over that cliff (with the appropriate fall down), then I will be looking more at it being a 3D RTS. Till then it is mostly just some art trickery.