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Now, I'll tell you about how I played through this mission. I played on Veteran difficulty, and it didn't work out on the first try because the Turquoise faction came to my lands too early with an awesome army. Also, the K'tharr units never dropped, so I initially leveled up a secondary hero who already had those units at the start. Eventually, I restarted, and the leveling went much better. I leveled up the following skills: units - tent - ballista - rapid-fire ballista; attack - shooting, fire arrows, battle frenzy; luck - magic resistance; logistics - pathfinding; education - the perk that gives 50% more rage at the start, and of course, the Horde's unique skill, Expert. In the first week, I cleared everything I could with minimal losses. On the eighth day, I recruited the Brown hero. At the end of the fourth week, or the beginning of the fifth, I broke through the garrison. I reloaded the battle a couple of times to achieve fewer losses, recruited the Orange hero, and then the fun began. After about a week or two, the Turquoise faction appeared with a very impressive army, and, in fact, throughout the game, it seemed to respawn again and again, even though it retreated from the battlefield. It was probably scripted that way. And in general, that first battle was the most difficult.
Now, a little about which upgrades are best.
1. It's very important to upgrade the Shamans into Daughters of the Sky; this is the first upgrade, not the second. And it's best to try to lose as few of them as possible because, of all the mage creatures, they are the strongest due to their chain lightning and infinite mana thanks to the Goblins!
2. Assassins must be upgraded into Leaders, and they should be placed close to the Centaurs and Cyclops to increase their initiative by 1, while the Daughters of the Sky already have good initiative.
3. Centaurs are better upgraded into Nomads; they have more damage, even if they have one less hit point than the alternative, plus they have maneuvering.
4. Goblins should be upgraded into Trappers; they have more health, and their traps often slow down the enemy.
5. Orcs are better upgraded into Warmongers because their horned brethren are weaker in health and defense, and Warmongers often save important allied stacks with their ability to distract the enemy.
6. Wyverns. Honestly, both upgrades are good, and which one is better to use in castle defense, we'll see below.
7. Cyclops. We definitely take Free Cyclops because Blood Cyclops are much weaker in damage, and even though they have a spell that reduces the creature's luck by 3, in my opinion, they still lose to Free Cyclops.
Now, I'll tell you about my castle defense tactics; many probably already know them, but just in case, it might be useful to someone:
1. Troop placement. I placed the Assassins close to the Centaurs and Cyclops to increase their initiative. I placed the Goblins close between the Cyclops and the Daughters of the Earth, and I placed this group in the lower left corner. The rest - Warmongers and Wyverns - in the upper right corner. Now, about the Wyverns, which ones to upgrade, I looked at the situation. The enemy always had Blood Cyclops in their attack, so I didn't dare to take them out of the castle, and therefore I upgraded them not into Paoka, but into Poison Wyverns. But outside the castle, I took Paoka, and they are good because they can hit several units at once (chain lightning effect), and they can also revive themselves by eating the corpses of killed creatures, just like Vampires. Poison Wyverns only restore 30-50 units per turn. But during sieges, they win due to their poison.
2. In castle defense, the "Terrifying Roar" battle cry, combined with the Trappers' traps, helps a lot. This way, you can hold the Cyclops at the castle walls for a long time. By the way, regarding them, to prevent them from throwing Goblins, I advise you to eliminate them first, and if you can't do it right away, scare them away with the aforementioned roar. This is very relevant because the enemy usually has more than 20 Blood Cyclops.
3. We support the Centaurs and Free Cyclops with Leaders and their ability to influence the speed of creatures on the initiative scale; this ability is stronger the more Leaders you have in the stack. So, at the beginning, we get rid of the enemy archers and mages. We also boost the Daughters of the Earth with Leaders, and they, in turn, perform a series of chain lightning attacks on the enemy, and with each subsequent attack, the chain lightning becomes more powerful in damage. As soon as the castle walls are breached, we slow down the Cyclops with Trappers and the roar. And in this way, we simply destroy the enemy: Centaurs - Cyclops - Assassins - Daughters of the Earth - Trappers. Using this scheme, I withstood sieges in which the enemy had a two or three times larger army. Of course, the AI always retreated