Here is the full list of all 10 Fort Defense trophies - there are 5 bronze, 4 silver and 1 gold.
1000 ships sunk
All episode levels completed with 3 stars
Level completed without using more than 5 towers
All improvements purchased on a single tower
One spell improved to maximum level
Level failed 3 times in a row
Level completed with last remaining life
Level completed in fast mode
Only one enemy ship attacked the outpost gates during the level
200 spells cast