The onion skin feature is working strangely with tilemaps. If you set it for two layers below and disable silhouette, the lowest layer is drawn over the top of the one only one layer below, meaning you can't properly see anything in the middle layer.
I'm experimenting with moving map design for my game to GC as opposed to the slightly clunky editor I put together myself. My game's maps use layers -- one for ground/walls, one for decorations, and one for creatures. It's hard to place creatures when you can't see the furniture you've given them...
Thanks for reporting problems here.