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I usually prefer the Shadow League, so I took a closer look at the alternate units of this Castle. The overall impression is rather debatable, but there hasn't been enough time to definitively determine the usefulness of certain upgrades.
So, let's start in order:
1).Assassin/Stalker - The stats are not significantly different in favor of the Stalker, but it's a melee unit, although it does have the Invisibility ability. It's a rather questionable alternative, considering how much hope was placed in it. At least, in the early stages, I prefer them, as clearing the spawn point becomes noticeably easier, and closer to the end, you can switch them back to Assassins, although Stalkers also look quite good in battle if you lead them with a Warlock.
2).Furies/Blood Witches - The alternate unit now has immunity to Curse, the damage range has increased, and it has more health (21 HP), but for now, I prefer Furies, as they have an initiative advantage of +2, which is quite noticeable both in skirmishes and in final battles, although if the opponent is not too agile, the Witches will be very useful.
It's difficult to say which of them is better, it's more likely that the choice will depend on the specific situation.
3).Guardian Minotaurs/Captain Minotaurs - The new ones are slightly tougher, but in exchange for 2 attacks, they received the Aura of Courage, which prevents the morale of surrounding units from being reduced by more than +1. They will most likely be preferable against Necropolises, but overall, they are equal.
4).Dark Riders/Shadow Riders - They lost their main ability, which allowed them to deal damage by reducing or removing armor (-20% defense per cell traveled) - and how thematic that glaive or escutcheon was in the rider's hands. In return, they gained the ability to deal, it seems, a quarter of the damage to all enemies they pass by - I don't think enemies will tolerate this for long; the old Lizardmen look much better.
5).Cave Hydra/Acid Hydra - The original has Regeneration (30-50 HP per turn), but the alternate has Acid Blood; otherwise, they are equal. The first option is suitable for clearing mines, while the second will fit perfectly into a general battle, where there will be no time to heal wounds, and spraying everyone around with acid will be very useful (25% of the Hydra's damage) - it's something like an infinite retaliation, and the leading hydra breeders of the League strongly recommend breeding them.
6).Mistresses/Twilight Witches - The new alternate unit has now decided to devote itself entirely to the whip and has abandoned shooting. The parameters have increased quite well (+3 to damage, +10 HP, +1 to intelligence and speed), and they also received Invisibility, which works the same way as with Stalkers. It's not yet clear whether Invisibility or shooting is better; perhaps Mistresses will be more useful, but I have some ideas for using Twilight Witches - I want to try taking an alternate unit with Letos and the Secret of the Spell and learn the missing Dark spells in battle.
7).Black/Red Dragons - Black Dragons have already proven themselves well and don't need any further recommendations; as for Red Dragons, the new ability doesn't yet give me a feeling of joy, especially with reduced stats and seemingly no immunity to magic.
In the end, we get that the alternate units of the League are generally equal to the original ones; I can't say anything definitive yet, as in some stages, some options look preferable, and in others, others; we should also not forget about retraining, although the Castle is still quite expensive, and it will not be possible to constantly switch large armies.
Now, there is only one thing left to do - play and continue to study them