or you can attack the castle early when all of your forces are still intact, then rally back when generals spawn, and have your biplanes join in for the quick kill
Main rule is: Never let out a general, if you are attacking a player at your level... Even if they are lower level generals, they will still slow you down. Your attack must be a hurricane. Forts are usually surrounded with "worth-to-sabotage-or-blitzkrieg" buildings, so I don't care about the level of the defending gens. I make sure they never come out.