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.
I was, indeed, thinking of taking one of the forts in the rear, and hijacking the signal system so that it would appear that nothing was wrong. Maybe not the rearmost, since that one is more readily observed from the evil kingdom itself.
Once the signal system was compromised, the forces of Good could attack or besiege the first fort in the series, while the party chewed its way toward them.
As for "modern" tactics, tactics always evolve to match the available military technology. If was was a military commander in an age of magic, I would try to learn everything I could about the magic I might be facing, and try to recruit some of my own. I already think about what mundane steps I might take to counter various spells. If my life depended on it I would study the ranges, areas of effect and capabilities of every spell that was known to be in play in the world.
As a DM I would probably assume that a 5th level MU cost about as much to train as it cost to build an equivalent cannon IRL (cannon was around earlier than most people think), and might be easier to take out. Given the large blast radius and the fact that 0 level troops are instantly killed even if they make their saves, I think this is way more effective than a modern 10 man squad.