Some years ago, In one of the Wargaming mags., someone had a set of rules
for attacking a Vietnam fire-base. I would go for something similar.
The defender does not know WHEN the attack will come.
he can-not have everyone on full alert 24 hours a day, but he
also can't have too few guards. It is doubtful that he will know when, or even IF,
help is coming.
In the Vietnam game:-
The first stage = the occasional bit of sniping, which might lead to some mortar firing. The
'attack' might not come for days, the fighting may escalate to the second stage,
or all Hell could break loose very quickly.
The second stage= Again unpredictable. Could quieten down or fully erupt.
The game becomes a war of nerves. In one of our games the US Officer player was so scared
of being Fragged by his own men, that he left the Fire-Base wide open to a VC attack.