If you are going to go all out and you think you may need to defend against a large force here are some mundane Ideas:
-Clear the forest for 100 ft in every direction
-Have a bolt hole or means of quickly escaping
-Have a stock of food
-Design areas in the fort to give terrain bonus to your players. (roughened terrain in the entrance, pits that trigger for large creatures, elevated firing positions, choke points in hallways where numbers mean nothing, ceiling drops, etc.)
Take a look at the star fort with several bridges to cross and numerous choke points it was able to hold off both Spanish and German invaders during the 80 year war.
I DM'd a large invasion of a mountain fort several years back and my players decimated an army of 85+ giants, orcs, and goblins. They dropped bridges as the enemy crossed, set up choke points for maximized fire balls and lighting bolts, started land slides, and used a stampede of large creatures (covers in burning pitch and carrying barrels of greek fire oil) to dishearten, destroy, and break the invading army.