To make a fort which will never, ever be taken by the enemy, you need to set up a game
in 'the black forest' as the name sugests, you start in...a very large forest, with a 
small village depending on what settings you have chosen(i personably would chose death
match, as you get lots of resources.) any way, you need to make sure you have a onager,
or something whcih can destroy lots of trees very quikely. you then start tunneling 
into the trees, making walls and gates along the way. put towers with archers behing
each gate, and make large areas where you can put castles, barracs, churchs ect and 
town centres. no matter how hard your enemy tries, they can never get through the walls.
This tactic will take about 1 hour.