I used to write about it and always used it :)
After defeating the first garrison, you need to select 7 Imps individually and fill it - the garrison will become vulnerable :) you can use it to deploy troops. If you don't do this after the first capture, you can forget about it - both the deployment location and the buffs for the final castle (there's an idol), and the free passage to the Necropolis. Moreover, after the orange player gets upset, he still won't be able to attack the garrison, and if the Manticores don't break through, this will be a retreat route and a place to capture the castle.
There's also a gold mine there (or an abandoned gold mine), guarded by Death Knights, but there are a lot of them on the hero (they don't get killed right away).
Added 23 minutes ago
You can't give up Logistics on level 4, because the map is large, the corners won't be explored, and enemies that have passed through the garrison won't be killed, and the zeroing radius will decrease, and the likelihood that they will break through the second city increases, so it's better to give up Magic.
Only the general slowdown/weakness effect is useful from Black Magic (or SA, which you can't go for initially, because they will kill you long before you get it), and Light only gives a general speed boost and Storm Wind.
I don't have enough general slowdown, but unfortunately, while the Demon melee unit is using the Sign. + Sudden Attack (haven't gotten it yet, but it will be soon) can keep the hero busy.
After the first replay, I got this (a week of alliance dropped):
https://c2n.me/406l8xz
all demonic skills and https://c2n.me/406lcAG
https://c2n.me/406leFl (battles with enemies in the hat, the cloak is nearby, but the hero was busy with enemies, now he's coming and will take it - it will be a Knight set)
Small map control - https://c2n.me/406lido (the space under the castle has been cleared, except for the Fires and Phoenixes, on the other side, only scouting is being done and only slightly cleared around the castle (the gem mine and the obelisk), + a little bit to the Machine Ring)
Reached the Machine Ring, took it, the rivals forced me to return. Here's an unclaimed Orc Shield https://clip2net.com/s/406llZp, I didn't have enough movement points to pick it up - I had to jump into the city - but I don't need the shield itself, because I have the Blade and Ice Shield (from the mages).
In general, the week of alliance gave me a boost, but it helped more to build new buildings. The Elf has 13 gold (=emerald dragons, just the color has been changed), I don't want to deal with their devilish chains :)
The Orc warrior caused problems, but here it seems like nothing special - I've already defeated several rivals, they came, left, and promised to return :)
This is a replay, after the ballista didn't work out the first time, it worked out the second time, but not right away.
In general, it would be nice to allocate a cell for Phantom - but the probability of Phantom in the 1st Demon Castle is EXTREMELY LOW :)
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I have an idea to take Eagle and Herbalism after Mentor and steal Phantom from the Mage, but I'll probably take SA, it's really needed here. Mages/Dragons don't give spells here, let's not forget :)
Added 28 minutes ago
although, forget about SA without BDP (you can't sacrifice Mentor, first I'll level up the heroes, then I'll change the defense to Darkness, when they will be able to defeat everyone and everything.
And the walkthrough is probably to first get Machines/BDP/Logistics/and preferably Mentor (if possible), collect artifacts and level up the assistants.
Magic, perhaps, is Chaos - to break the catapult, or Herbalism/Summoning - if you're lucky with Phantom, but the chances are small, instead of attack