I love this game...
However, I feel that the game tries to get you to do two completely different things at once. Firstly, you are playing for the high score which in this case is the fastest departure time. At the same time however, the gun/turret upgrades take a good while to max out leaving you with two choices.
a) You can play as fast as you can to get the high score while missing out on the cooler high tier upgrades
b) Lose your chance at a high score but explore everything this game has to offer.
Also, the repair machine you could by with the fort truck was somewhat useless in my opinion since you only needed it 3x in the entire game and the forts upgraded fast enough already. Perhaps in a sequel (which I would love to see), their could be things that attack the fort forcing you to repair it on the go. These are of course somewhat minor problems in an overall great game. Keep it up!