In Defense of the Watchtower
I'll just say it, I think the Watchtower gets a bad rap. This is the dreaded scenario 6, for those who are not familiar, and it involves placing a tower in the direct center of the board, and the only victory condition is to be the only person in it. Since scenarios are randomly rolled, you must always prepare to fight this lovely battle.
The general consensus on why the Watchtower is a bad scenario is basically that it's either too hard to kick an enemy unit out of the watchtower, or that it requires building an army which can effectively assault/defend the tower. Because of this, people generally don't like playing the scenario.
My Take on it
When you play a game of Fantasy and you are playing random scenarios, you need to keep all 6 in mind, and two of them don't use Victory points to determine a winner. This means that every army I have can either hold or defend the tower well, or both! Winning any game of Fantasy involves strategy, so lets briefly go over some tips and tricks for the Watchtower.
First tips for holding the tower. Don't think you have to start in the tower, if the only unit that you have to take the tower with is a unit of 10 Gnoblar Trappers, don't start in the tower! The reason for this is that while you cannot march to gain access to the tower, you certainly can charge it. If you are going to start in the tower, be sure to have a unit that can survive at least one round of combat, since the opponent will probably charge on turn one. In addition to having a unit that can stick around, make sure the tower is in both your General and BSB range ASAP, this will ensure that if they do lose, they will probably stick around. Immediately, march a tougher unit right up behind the building, so that you can eject the smaller unit in favor of your tougher unit. Now all you have to do is keep that unit alive long enough to win!
If you're attacking the tower, you may feel like you're getting the shaft, but actually, I prefer to start outside the tower. Many times your opponent wont have a unit that can adequately hold the tower, so they will opt to leave it empty so that you will have to wait until your turn 2 to get into the tower, which you should have no problem doing. If you aren't able to waltz in on turn 2, you need a strategy for killing whoever is inside. First of all, everyone (except dwarfs) has access to the Banner of Eternal Flame, which not only makes trolls and hydras run for the hills, but it's also wonderful for allowing you to re-roll wound rolls when assaulting a building! Since most people take this banner all the time, you just need to be sure it's on a unit that is good enough in combat to beat up some jerks in a tower. In addition to close combat, you really do need to focus on the tower with everything, magic is especially effective since it is not affected by the -2 penalty for being in hard cover, blast weapons as well will do D6 automatic hits to guys inside, and they will be at the higher strength for stone throwers if the center of the hole is over the tower, so they are quite effective siege weapons.
Ultimately, the key to winning the Watchtower is to saturate the defenders with as much as you can, and prevent the enemy from getting reinforcements in. Typically to do this, I use charge redirection tactics and just put anything I can between the opponent's army and the tower, while I siege it with anything I can. This creates a very different game of Warhammer, but it is not a game that is unfair and no less tactically sound than any other. I hope you look at the Watchtower with renewed hope and roll some 6's when determining scenarios!