I am going to write a Wiki article about this as well but I also want to publish it to all the mappers out there to read.
I recently talked to Ablu about mapping guidelines. He told me a few things I didn't know which make perfect sense but aren't realized. It's about the use of Layers.
Fringe
The Wiki is pretty specific about what to put into the Fringe layer.
Still it seems nobody gets it (neither did I). That's why I am going to explain it more clearer here.The "Fringe" Layer itself is for oversized tiles which have to be drawn in front of or behind moving objects, depending on their location. This sorting takes up CPU, making the game run slower. Thus, you should only use this layer for the tiles which have to use it.
The Fringe Layer should only contain tiles which are larger than one tile and appear behind the character if you walk south of it and in front of the character if you walk north of it.
We all know the Woodland_ground tileset, and how it is used. Stones in the Fringe layer etc. That is wrong. None of the tiles from the entire Woodland_ground tileset belongs into the Fringe layer!
Fringe is only used for tiles which are bigger than one tile and have to show behind the character if you are south to it and show infront of the character if you are north to it! Trees belong into fringe layer. Thats a perfect example.
So some will ask, "Where do I put the stuff I had in Fringe now? It doesn't work in Ground becaus it is transparent!"
The thing is that you have to create more than one Ground layer. Ground, Ground2, Ground3 etc. As many as you need to make your map work. You don't automatically have to create 3 or 4 Ground layers but in most cases you need at least 2. (Ground, Ground2).
Over Layer
Walls should be in the Over layer just to make it look better. (I didn't know this either and I made tons of errors not knowing that :-/)
As you can see here I'm standing next to a wall with a staff and that makes my character wider. The staff is shown above the wall. That looks rather strange but aren't we all used to that?
The better way would be if the wall was in Over layer so that it would look that way.
This works with the Tulim and the Woodland tileset
But take care: This will not work with the Snowwalls! If you do that with the snowwalls you will have troubles with larger hats such as the christmastree-hat
it will be cut off.
Conclusion:
Most of the maps do not realize any of the things I wrote above, some are partly correct. I would nevertheless expect new maps to comply with these standards. It's not like they are new standards it's just a reminder how the layers are intended.
I've dedicated myself to fix all of the current maps. I have no idea how long it is going to take or when I will be finished but I guess it will take forever.