Speaking of Watchtower, you can use Detachment rule for a little trick. In the Official Update Version 1.9, p. 12, one can find this little nugget of information:
Q: What happens if a unit that is assaulting a building is itself charged? (p127)
A: The assault on the building is abandoned (move the unit that was assaulting the building backward 1”) The two units outside the building are now engaged in combat instead and will fight a round of combat this turn.
This is tailor-made for our Detachments. If an enemy unit assaults a building which contains a Regimental unit, any Detachment could declare a Counter Charge action, which is resolved once the enemy has successfully completed its charge against the building. As per the FAQ, the assault is now abandoned, and the enemy has to battle it out with the Detachment first. As it is likely that the Detachment will be quickly disposed of, you can rinse and repeat with a second Detachment, if necessary.
Remember, however, that the Detachment will be steadfast, and with the General/BSB in the area, that should be enough from keeping the Detachment breaking, as long as there is a single model left. If you also add a Captain (Hold the Line) to the Regiment, the Detachment would become virtually unbreakable. To enhance the chances of survival for the Detachment, you could also add a WP to the Regiment, and aim to get the Shield of Faith (5+ ward in CC), and perhaps Flaming Attacks (to cause Fear in war beasts and cavalry).
A Regimental shooting unit could declare a S&S the first time, and if the Detachment is destroyed, could then fire at point blank range, during the Shooting phase in its own turn. Of course, if the enemy tries an second assault, a second S&S may not be possible (except for pistols).
This trick may buy you two extra turns, before the building can be assaulted. This is particularly useful, if holding the building is a victory condition. Of course, this only works, if there has not been a successful enemy charge against the Detachments themselves.
