It is my opinion that the Tower is the worst of the Major Arcana cards. It depicts bad news, a terrible happening or event. It gives warning that something is going to happen, whether it be physical or emotional, the news is shattering - thus the tower. It should depict a streak of lightening in the night sky striking the tower and stones falling to the rocky shores below. Its all about how you prepare for the news or event that makes the card so unappealing to those having the Tarot read for them. You can either take the news and prepare for it or you can suffer the consequences of not readying yourself properly. Either way the Tower is meant to make you feel like you are falling from a high place out of control with no way to stop what is happening.
I'd start with a dark sky, then add lightening. The Tower card can be read many ways so it will be up to you to render it in a way that allows the reader to "see" many sides of the story.
I will help as much as I can. Good Luck!
"Creativity emerges only when the imagination is given the freedom it deserves."