Regular LOS rules are used. This makes it a lot harder for attacking figures to hit defenders.
Don't you find it makes it almost impossible? For a figure on the ground to see a figure on top of two castle sections behind a battlement the figure on the ground needs to be a good 5-8 spaces back from the base of the wall (assuming normal sized figures). For some figures that is out of range. However, a figure on top of the castle can use the aiming point and see a figure straight down 1 hex away.
We obviously house rule that the figures on the ground are attacking when the fortress figures are peeking over the wall since in a real battle everything is happening together, not in neat little turn/round sequences.
Even with this house rule we still find it nearly impossible to attack a castle even with the attackers having 50% more points in their army.