Here is a small list of ideas to improve this.
1. Animate more! with the last level that had lava you should of had the lava animated.
2. Longer levels that scroll as you go along.
3. Enemies! One example could be in the castle areas with lava have those fireballs jump up every one and awhile. another example you could have bullet bills being shot at shy guy. (Make sure that specific enemies can be killed by jumping on top of them!)
4. different music! when you change from one area like outside to inside the inside music should be the castle song.
5. have a level select screen
6. make collision detection alittle better. sometimes when i got close to an edge i would fall off when i should have not.