Interesting. Does the selection works fine on all the layers ? (I mean also on tile that are higher than the basic floor)

I'm not really sure how to explain it, but your solution won't work when you add variable heights to your tiles because you can't know in advance what the height of the tile will be.

For example, a tile in (x,y,z) = (0,0,0) and a tile in (x,y,z)=(1,1,2) should be drawn in exactly the same space on the screen, the same for (2,2,4) and (3,3,6). (it might be different numbers depending on how exactly your solution works, but nevermind that.)
When you point on that 2d location on the screen, you have no way of knowing if the tile in (0,0) is drawn there, or if it is a tile in (1,1) that is 2 high, or a tile that is in (2,2) and 4 high etc.

EDIT: Yeah, alpha blending in A4 is incredibly slow. Just switch to A5

Lets put it like this, I have a tile in (0,0,0), it is drawn on the screen in coordinate (300,0).
I also have a tile in (1,1,0), it is drawn on the screen in coordinate (300,32).
I also have a tile in (2,2,0), it is drawn on the screen in coordinate (300,64).
I also have a tile in (3,3,6), it is drawn on the screen in coordinate (300,0).
I also have a tile in (4,4,0), it is drawn on the screen in coordinate (300,128).
I also have a tile in (5,5,6), it is drawn on the screen in coordinate (300,128).
I also have a tile in (6,6,6), it is drawn on the screen in coordinate (300,192).
I also have a tile in (7,7,6), it is drawn on the screen in coordinate (300,256).
I also have a tile in (8,8,6), it is drawn on the screen in coordinate (300,288).
I also have a tile in (9,9,6), it is drawn on the screen in coordinate (300,320).

What happens when I click (300,1), do I get the tile in (0,0,0) or the tile in (3,3,6)?
What happens when I click (300,96)? Which tile do I get?
What happens when I click (300,320)? Which tile do I get? Keep in mind that according to what you just said you would get the tile in (8,8,6).

Here's how it works:
Lets assume I have a tile (X,Y,Z), and lets also assume the because of its height it has been drawn to screen coordinates (A,B). If it had a different height it might have been drawn to (A,B-32) or (A,B+32), so I can't really tell that this is the tile I want. (so far I think you agree with me.)
Now, what I do is assign each tile a unique color using a simple formula, for example lets say our tile (X,Y,Z) received the color (r,g,b)=(X,Y,0).
Now, as I draw the tiles to the screen, I also draw their color counterparts to a side bitmap. Since I'm using the same drawing process, if I drew our tile into (A,B) on the screen, it should also be on (A,B) on that bitmap.
This means that if the user had clicked (A,B), I can check what color (A,B) is on the bitmap and I will see that it is colored (X,Y,0), from that I can deduce that it belongs to the tile in (X,Y,Z), so I know exactly which tile I chose and I am now happy
Using this method also helps solve the case where two tiles are drawn to the same coordinates (one is hiding the other), because your drawing algorithm should already take care of that and make sure that the closer of the two tiles is the one that is visible (and is therefore that one that will be drawn both to the screen and to the bitmap), and now you are twice as happy

You have two possible approaches:
1) Store one giant bitmap - since the map rarely changes, and if it does then it only changes in a small area, you'll rarely have to redraw it. Sure, it can be quite big, but at 64*32 you have can have 256*256 tiles in 128 megabytes, which isn't much for modern games.
2) Use a bitmap as big as the screen and draw to it every time you draw to the screen. This means that drawing takes twice as long, but the extra memory usage is negligible. (And for drawing isometric tiles, drawing time should not be considerable enough that you care about doubling it...)

I'm thinking of an isometric view too for the world map.. However, I'm thinking of fake-isometrics instead of the one you are discussing here. By fake I mean using rectangular coordinates as a normal 2d game would use but tiles are 16*8. I guess diablo 2 uses that method. 1 rectangle contains 4 triangles.

@J-Gamer: Sorry, typo, I corrected it.

@Hyena: It seems to me that this could have more problems than a normal isometric view. In a normal isometric map you're basically just rotating a square map 45 degrees, but figuring out the grid location is just about the same.

One note about Diablo levels, they don't have the problems with height because Diablo levels are flat, they don't have any height at all, you won't see a hill in Diablo, go ahead and look. A flat map is uber easy to plot and get mouse locations on, I am doing that already, even with small hills, without trouble.

I have considered not even bothering to worry about having hills in my game as most of the really successful games I have noticed have been flat isometric games, they get away with this by having a lot more detail in their game so you're so busy admiring how nice it all looks you don't even notice how flat everything is. There are some Diablo levels that appear to have height (like in a few with stairs) but this is just an illusion, they're still flat.

I may just stick to a flat map with small hills like I posted, there isn't really any big need for large hills anyhow when I start thinking about it. It's certainly not something I wish to stress over.

Edit:

If anyone's curious, here's what I have created so far: isomed.zip
(Windows executable, compiled for use on i586 or better CPUs, I also used ImageCfg on this so it will be forced to only use single CPU on multi-cpu platforms.)

{"name":"604028","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/4\/9\/493bc326fe32f4666ba1e467e44e9506.png","w":800,"h":600,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/4\/9\/493bc326fe32f4666ba1e467e44e9506"}

it's fairly basic at the moment, you scroll the map around with your cursor keys, left clicking and dragging around the terrain defaults to "drawing hills", right click flattens them again. Road drawing mode is similar, just left and and drag your mouse around the terrain to draw roads, it will automatically connect up the roads similar to the way Simcity 2000 does it. Right clicking will erase roads and automatically "disconnect" adjacent roads. I haven't added in code to put roads on top of the hills yet, you can put one on the side of a hill. I'll get to that next. The map is currently 50x50, although it can be any size, this is just one I made up for now. I want to make all this configurable from a menu in the future.

That interesting because I was playing Locomotion (newer version of Transport Tycoon for Windows) and I noticed they had an option to toggle gradients on/off to "smooth" out the edges and that got me to thinking about trying that myself. I also want to redo my graphics so there aren't grid lines and make them something you can toggle on/off. I'll also probably redo my roads so they're done more the way Locomotion does theirs. I', also thinking about making the tiles so that they are higher when you raise them, but I'm not certain about that yet.

I'm thinking about just allowing the hills to be only a couple layers high, they don't really need to be too high anyhow. I had noticed that the tops of hills being the same shade as the land really took away from the look of them being higher, I may just program it to have lighter colours for height, not sure how I will implement this yet, but it does look nicer, now that you have done something up, I really like the looks of that. Add in some trees, brush, real looking grass etc... and it could get quite nice looking I think.

I originally had the editor so you clicked on a tile, then places it, but when trying to make large maps this got VERY tedious so I looking into ways to automate the process so you could just draw them. What I do is have the tiles organized so the corners of the tiles are like a binary...

 1
8/\2
 \/
 4

The main base tile is tile #0, zero meaning no corners are raised. If the top corner is raised, it is tile #1, the right corner, it is #2, if the top and right are raise, you guessed it, #3 (1 + 2). Etc... this way, when I raise a tile up by left clicking it, obviously all 4 corners are raise, all I need to do is take the value of the surrounding tiles and logically OR them with the corner that gets raised, so if I click a tile, the tile above it (Y-1, X-1) has the southern most corner raised up to match, so I take the value of that tile, OR it with 4, and the result is the new tile# that goes there, this makes it really simple to raise all the surrounding tiles up and make a hill. The code to do that is very simple:

I check to make sure the tile the mouse is over is greater than zero (I set tile_mx and tile_my, the tile the mouse is pointing at to -1 if it is pointing off the border of the map).

Tile #15 is all four corners raised as you may have guessed. I want to change that part of this so it instead sets the "height" of that map tile.

The rest of this simply checks to make sure it stays within bounds (less than MAPSIZE, greater than zero) and then ORs it with the appropriate number for the corners effected. Road tile pieces are all greater than 19, which is why it returns if the tile # is >19.

And of coure, lowering the terrain is similar, you lower the main tile (set it to zero) then logically AND all the surrounding tiles so the connected corners are all lowered, which leads to the proper tile #s being drawn.

I do something similar for the roads as well, except with them I call a recursive function which adjusts surrounding roads so they automatically connect up. The road pieces are organized in a "binary" fashion like the terrain.

I may redo the tiles so that they are all WATER at the base location, and you raise up LAND when you click, so you see islands when you click, otherwise it's all water when you start.

That could probably be improved, but it's just my first draft of that function, I don't like all those "if"s in there, but...

Oh I seen what you meant. My scenes are lit by the game though, could be interesting to try though.

Edit: While looking over some other websites a possible solution came to me with regard to selecting tiles that are several layers high. Once you select a tile, you could then compare where it is drawn on screen with where the mouse is pointing, then maybe do some maths on the distance and esimate how many tiles "south" of where the program selected you are actually pointing, perhaps "walking" one tile at a time until you get the correct one. Or in the case of valleys, walk north. I guess you would test one tile to the south, one to the north, whichever brings you closer you would continue to "walk" in that direction until you get the correct grid location... I'll have to play around with this idea, I was going to not bother with higher terrain, but it bugs me too much to just drop it like that.

Edit: Upon further reflection, I have come up with a better solution that I think will work (I don't think the above solution will). I based it off of the ray casting concept where you draw a line from the viewer into the scene and see what it intersects with. Well, with isometrics the best way I could come up with that is similar would be to start a fixed number of tiles below the mouse position, then test that location to see if the tile position matches. If not, then move one tile north and test it again. Keep doing this while the top of the tile being tested is below the mouse_y position. Once the top of the tile is above the mouse_y than you have the right tile. If you start at the bottom of the map and work your way up, you will always get the right tile. The number of tiles you start below the mouse position will depend on the maximum height of the hills on your map. You could of course start at the very bottom of the map, but this would probably just waste time. If you have really large maps like I have, you could also just start at the bottom of the visible map.