Holding Shift on the keyboard while placing a tile applies it how it appears in the chipset, regardless of different surrounding tiles... For instance, let's say you place a 16x16 water square surrounded by grass. Normally there will be an edge of grass and earth around the water. But if you place the water tile holding Shift every pixel of the 16x16 square will be water.
Charsets are cute ^^
I think the map would look better if you held Shift while applying the floor around walls and ceilings. Also it'd be best to put the floor's "round corners" in the Upper chipset area and remove the white right angles.