From the Amphibian Abstracts blog: "So far, my 8-bit style platformer has been nothing but a frog walking around. It was too safe. There was nothing bad for the frog to hit. To make a proper platformer, there must be some enemies. In response to this problem, I added some toads.
To accomplish this, I first added an Object Layer to my game map in Tiled. On this layer, I placed some tiles that were pictures of the toad. You don't actually have to use tiles here - the objects are only placeholders for Sprites that will be added in the game. You could just use rectangles or whatever, but I think it's nice to be able to see the enemies in the map editor."