May I suggest speeding up the animation? Also, the second arrow's shadow from the left looks out of place.

 need to reverse the speed of the arrows, make em pop up in 1-2 frames, and slowly reset.

  nice! really liked the shadow animation. 
You could've also animated the shading of the spikes, they are light on the top part even when half-burried, so it seems like just pre-shaded sprites going up instead of spikes actually receiving lighting as they go up.

 Thanks.

 Interesting. Rather simple, but I like the shadows.

 lol never thought of that.

 the only thing i have against this is how slow the arrows raise. anyone could avoid that trap :P

looks awesome though!