During a game I've noticed that when using spells such as Power of the Tower there is no indication on the UI circle as to where the spell can be placed.  Therefore, if the spell is activated too close to an edge or in an unbuildable spot the character attepts the spell only to have the words ABORTED appear on screen after the spell animation runs.  The problem with this is that the player wastes time attepting a spell where it could not be used and loses the ability to attack and play normally during the duration of the animation.  Since it is so difficult to determine where the spell can be used, i suggest THE UI RING THAT APPEARS AFTER CLICKING ANY SPELL THAT DEPENDS UPON TERRAIN SHOULD ALTER BETWEEN TWO COLORS DEPENDING ON WHETHER THE SPELL CAN BE ACTIVATED THERE OR NOT.  So, for example, when Power of the Tower is clicked the ring appears green if it can be used on that particular spot but red if it cannot due to the terrain.  This will help players avoid wasting time on a spell that cannot be deployed.  Also, the animation should not take place unless the spell is able to be carried out.  I realize this a request more than a bug, but the effects of not having this sort of system can be so devistating that it seems almost necessary to me, thus I described as half bug.  

EDIT:  Also, while having color being the only indicator of allowed spell placement would only require one attempt before the user understands what the system is conveying, having the text "Spell cannot be applied here" written in a small, light-colored font on the bottom of the screen somewhere when the circle is over an area that will not work will help the user to more quickly pick up what exactly is happening.

ps - the ecessive use of caps lock during that one sentence was to quickly highlight the most important part of this feedback post were you to simply scan over it, not to appear as if I was yelling that information haha