Looks pretty good so far. Doing super small sprites is a challenge. I went the other direction and used larger heads with smaller bodies to display my characters. I hope you dont mind me posting a mockup in here it might give you some ideas.
You might be able to get away with doing one wall. Kind of like the side of the island I did here, that would make it a little easier for people to program because they would only need to worry about a few tiles, instead of doing all the different ones. (I have a better example if you need it)
also maybe think about doing a few floor textures. A you can use it to break up the monotony, and B you could use it to set up a grid system, which can be troublesome if the world generates large open areas.
As for the Sprites I think you did a good job in making things look different IE. centaur vs Ape vs Demon. the specific ones in that family are a little hard to differentiate, like the centaurs, especially if they are displayed smaller on screen then what you have there. It might be better to stick with one Type of each, although I guess that might be at the discretion of the game designer, perhaps in their game centaurs use bows so they use the sprite with the bow where someone else might use centaurs with axes.
and another thing is think about the location monsters will most likely be at, and when you draw them, do it on the base color for that areas tilemap. In my above example the main character is very readable, but if he is in an area that is very blue, then he would be much harder to see. That might be a problem with the sprites later on. I bet the Dark Ape, would be rather hard to make out against the cave floor, but if you were in a snow world he would really pop at the player.
