When I play a normal level, I often spend several minutes looking at the map, and planning where I'll put my various defenses, as well as in deciding which defenses would be best.
When I play a challenge level where there is no choice of defenses to stock, there's no way to pause the action and just look at the map before the clock starts ticking, and no way to pause the action during the game that still allows a view of the map.
It would be most helpful if there were a way to pause the game and just look at the map for a while, to decide what I wanted to do next. If that's too much like cheating, adding a way to let me look at the map for a challenge level before the action starts is good enough.
In a download or web game, it's easy to take a screen shot or several and then look at the map outside the game, for planning strategy or even zooming so that I can try to find small hidden objects. It's not nearly as easy to do that with an iOS game.
Please? I for one play for the mental exercise (puzzle) aspects, and just put up with the time pressure aspects. I don't get pleasure from being hurried. (heh, a couple of years ago I published an article in Casual Connect magazine about relaxed mode game options)