Suggestions on what to add to my app

I'm making a free app for android and ios, but it isn't very "fun" right now. There isn't much to the game and I'm stuck on what to add. You can play the game here, and any suggestions or comments would be greatly appreciated.

 

I made a game very similar a while back, you can play it here if you want to get some ideas

I think it would be nicer with some background music and prehaps some more ineresting graphics. There was one released on the play store with random events where things would start moving or the screen would go upside down etc to spice it up a little.

I think a little colour would be quite nice too

But a nice game, well done

 

Thanks! I was thinking about adding some background music, but I cant really seem to find the type that I'm looking for.. There will probably be some different graphics for the towers in the future but they are there until I get those .

 

Color
Music
Sounds Effects
variation of speed
A larger view
the yellow pill things can spawn in walls (Fix maybe)?
different characters or colored balls
a money system to buy the above ^
increasing difficulty

 

Thanks for the suggestions, I added multiple different characters that change the ground accordingly. May add different colored or textured towers in the near future. Music and sound effects are close as well