Taking out towers is a tough job if not done with care. Towers reek havoc on the life of a unit. They give great view to your opponent and serve as a barracade. The first move you make in a seige is to obliterate the towers.
On land you will want to use your Ballistas or Catapults. They have a slightly greater range than the towers do. One warning however. For the most part, the instant you attack a tower, your enemies troops are on their way. Be prepared to defend that Catapult. I advise using more than one at a time.
If you are in a sea battle and there are towers on land an effective tool to use are battleships and possible destroyers. The trick is to get the first hit in. You will probably recieve a couple of hits but at least you get first strike. If you use two or three upgraded battleships it is possible to not even be hit. What you do is order the ship to move toward the tower. The tower will fire and will generally miss the moving target. Then immediately order your ship(s) to attack it. It will stop and fire giving you the first hit.
Drew's Little Heaven/drew@nbnet.nb.ca/last revised, July 9th, 1996