Tiles by themselves are not that bad (I like the wall, it's very NES-ey <?>). Try doing some borders for the walls and grass and they will see much better. And those dark-sections (some kind of ceilings?) that are that common in RTP.
Its a cave, although a strange one. Yes, i like also how looks the wall. For now im not doing any borders, i will make them for a more advanced version and when i improve them. For now its enought, the prototype will have that.