I would heartily recommend reading Sun Tzu's "The Art of War" which gives lots of good advice.

Also: Time Spent in Reconaissance is Seldom Wasted!

If possible have some mounted scouts who can keep a look out for those enemy hordes. Follow all of Hawks tips above.

Look for choke points where the enemies abilty to attack you is hindered by terrain/fortifications

Cannon with case-shot at close range is good for breaking up massed enemy infantry.

It's never to late to RUN AWAY!

AWM (Wargamer for lo! these 30 years)