I am not saying that I am one of the best players, but I will challenge anyone who dares to say that I am not!
I use only watchtowers, but it may be due to the fact that I play only Vanheim. My forces are extremely mobile and mostly undetectable, so I can usually relieve the siege of any castle before it falls... or teleport a welcoming commitee for those who dare storm it.
Why would I even consider another castle? My troops needs little in the way of ressources, I fight mostly with mages, I like to build a castle in every province, and I don't rely on walls, tower archers and PD for defense.
In my present game, there are four races that look like possible winners. One is an underwater nation, the other three use watchtowers.