Tower's played a lot like Golf Solitaire. There's a waste pile and you are trying to move all the cards from the tower into the waste pile. If there are no cards in the waste pile, you can move any card you choose into it to start out. Once the waste pile has a card on it, you have to place a card on top of that one that is one greater than or one less than the card currently on the top of it.
There's also a deck that you can deal a card from on to the top of the waste pile. You usually want to do this if you don't have a card that can be placed on top of the card that is currently on the top of the waste pile.