Towers are the strongest point of a castle. They can be easily defended even if the walls of the castle have been demolished and, because they command a view over the entire tabletop, they make ideal positions for archers and war machines.

You can place as many models on a tower as you wish within the limitations of the space available. Models that are going on top of the tower can simply be placed there, but if troops are going inside the tower (to shoot from windows for example) it is more practical to remove the models from the table and make a note of how many there are and where they are shooting from. You can fit as many models inside the tower as you can place on top.

A single war machine along with its crew occupies the entire top level of the tower. Note that you cannot move war machines if they are on the top level of a tower, except to turn them to face the direction in which they are going to fire. Troops occupying towers can shoot in any direction, either over the ramparts or through arrow slits.

Troops on the top level of a tower are vulnerable to missile fire, but they do receive the benefit of being in hard cover because they are protected by the ramparts and so are -2 to hit with missiles. Troops inside a tower are safe from harm except, obviously, if the entire tower collapses.