@Lord Aiden
Ok before you can understand the battle you must understand the history of the land,
Very briefly the contient is divided into 5 regions of influence, each one controlled by a prince.
Prince Azurian controls the north-east territory, a very ore rich and somewhat economically developed part of the country, his intention is to dominate the rest of the regions and create a kingdom under his rule alone, obviously you don't want that to happen, but the Prince has already incited a civil war. Your goal in this mission is to invade his domain and destroy the royal castle; a symbol of his power as well as the location of his current whereabouts.
This is a rectangular map; the width is much smaller than the length. You start at the bottom while the royal castle is directly across from you on the top of the map, the map is an uphill struggle, and the castle's primary defenses are 1) Flame towers; these shoot at and burn both ground and air targets, 2) Blue dragons, 1000hp dragons that are extremely effective against any air units, but are also pretty good against ground threats as well, 3) units known as marksman; basically a very effective archer with a really good range, 4) Azurian Guard; a 1350 giant human unit that has resistant skin and an ability that reduces damage from magic 5) Astral keeper; a void walker like being that is ethereal the entire time but can do huge amount of magic damage to both land and air units, can also cast hex and soul burn.
The castle has inner and outter "zones," the defenses I've described are basically what's guarding the castlem in other words the inner zone, the outter zone has all kinds of human units present, these are less powerful but more numerous, you have to go off against 2 major bases while are extensive in order to make it to the castle, and you can't just take your air units and fly over there cause of the dragons which will destroy any air unit that come within 1500 range of the castle.
You have a 250 food limit in this mission. Mission starts, you have 9 minutes to destroy a small mining outpost, take their gold mine and establish a base, (you have to do this before dawn) then you have to fight off various forces and make your way through the defenses (think of it as twilight of the gods, only you're playing as green, and you don't have archimonde but 3 powerful heroes instead. Once you get past the middle of the map, the Prince gets scared, he begins building this magic devise that basically creates a black hole at a particular location; and therefore destroys a quite a large are. It's survivable but its easier to just go and destroy him before the weapon is fully charged. Depending on difficulty you have anywhere betweeen 45 and 15 minutes before the thing is fully charged, at that point you gotta destroy all the defenses and breach his inner zone. You will get some reinforcements once you reach certain points, finally when you are within a good range of his castle the prince will start going crazy, he will use mass teleport like spells to gate in tons of loyalists, attempt to bribe you, have his harvesters attack your forces, even have his workers convert to heavily armed militia to fight you, at that point you gotta do everything and anything to destroy the castle.
So yea that's basically it.