Layering in old versions (pre-1.9?)

In GD 1.00, the following layers existed (from top to bottom):

Portal - the right part of portals is on the highest layer and renders above all objects
Ground spike - the ground spikes render above everything (except for the right part of portals)
Default - this is the layer for most objects, such as blocks, pads, orbs and the left part of portals
Blending - this is the layer for ground spike deco and chains

In each of the layers, the objects that are rendered last take priority (and usually that matches up with their order in the level string)

It's worth noting that this layering system had some changes made to it before 1.9. In 1.10, the decorative grid block became its own layer between Default and Blending (non-solid black objects from future versions go on this layer). In 1.40, the Ground Spike layer stopped existing and got merged into the Default layer.