There will definitely be stocks of water, food etc. that can be sabotaged (even the nice piles of boulders for the giants could possibly be rendered inoperable somehow, or chains of command to disrupt, hijack their signalling system to send false orders of retreat, cause morale rolls causing defenders to evacuate. The forts are in a string - but some are more like bunkers, others (or only one?) are where plans are laid and reinforcements sent from. Capture the tallest fort and destroy those adjacent with siege engines.
I think taking the rearmost fort first could cause a lot of worry as the others realize they are cut off from escape. Good or bad idea? They will fight harder but may attack to retake it, allowing the players the defensive position.
But yeah, this is assault under time pressure.