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We should apologize to the esteemed Shogun. I found the "Unicorn Bow" artifact on the map, which removes the penalties for ranged units. However, even though some limitations will remain because the enemy is in a Castle, the spearmen will still hit twice as hard. So, there's a chance.
I replayed, and the script works from day one; the enemy starts developing a Castle, fortunately, without capturing anything. By the beginning of the second or end of the third week, he fortifies the walls to a Castle level and hires Lava Dragons. The number of dragons increases by +2 per week, starting from the second week, so you can't delay too long.
But attacking immediately isn't a good idea either, as the army isn't strong enough. Therefore, it's best to combine army building and speed. Fortunately, there are plenty of mines and places to hire creatures. I've only explored the Dungeon so far; I'm attaching the map. It shows the mines, and there's some artifact in each corner. You can take a large amount of gold (+1000) and a Clover from the hero who runs around the Dungeon, allowing you to defeat him with minimal losses of 8 Gnomes on a bear. You need to run through the mines first and collect a few artifacts. I leveled up twice, improved my Chaos magic skills, and bought machines. A lucky shot from a ballista destroyed 20 of his ranged units, and I took out 14 with a lightning spell; we can survive.
On the map, levels 1, 2, and 3 represent the levels of creatures in the Gnome Castle: Defenders, Spearmen, and Gnomes on bears, respectively. Red circles indicate hero stat boosts, and stars indicate stables (named after a horse). Where I marked "mm" is the Machine Workshop; next to it is a red key, which opens the gate at the bottom of the Dungeon, on the left, where there's a horde of Bone Throwers. I haven't found a creature upgrade building on the map yet, but there should be one; if I find it, it will be very useful... [some time later]... no, there's no creature upgrade building; I've searched the entire map.
Therefore, you can prepare and then take the Castle. As you can see from the map, there are places to hire creatures. 56 spearmen per week.
Screenshots from the end of the third month. I haven't gathered half of the creatures from the summoning places. Earth Shock deals 1200 damage.