I propose modifying the freeze spell in the following way to restore balance against the clearly overpowered inferno tower:

The user taps and holds and is shown a radius and a duration time; by moving the finger the radius can be expanded to freeze more area for less time or concentrated to freeze a smaller area for more time. I propose at highest concentration (smallest radius) the duration be 100% longer (2x) the current duration for each respective spell level.

I also suggest reducing the cost and build time, perhaps to match that of the lightning spell.