- How to defeat Galtran?

Honestly, I haven't played against skeletons on XL maps, but I assume that whoever has more castles and they are more uniform will win. Regardless of the race.

From personal experience, I can say that the probability of winning against a serious opponent playing Undead on XL maps is approximately 20% out of 100 possible outcomes.
And these 20% fall on the early stage of the game (Necropolis is a neighbor, develops faster, gathers forces, and attacks). Otherwise, it's impossible... the longer the Necropolis is on the map, the less chance there is of defeating it.
And the number of identical castles won't have a significant impact when playing against Necropolis... the Necromancer, throughout the game, practically doesn't lose anything, but only accumulates and accumulates skeletons, which cannot be said about the other castles; there, you always have to lose something, even if it's a small part... and you won't be able to build a really strong army, even with three identical castles, to match 4-5 thousand of those Galtran skeletons :) ... besides, you first need to FIND, CAPTURE identical castles, and if they are neutral, then DEVELOP them... all this usually takes a lot of time, effort, and nerves :) by the time you get all this, Vidomina will already have at least 5 thousand skeletons, about 5 utopias will have passed, and there will be a fully formed hero with portal and teleportation magic... he will simply crush all the cities like a tank... and you will have to give them up :) ....
THAT'S the reality