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The only thing that Heroes 4 lacks is a well-thought-out balance, which is quite difficult to achieve. Currently, everything depends on the size of the map, the density of neutral armies, and the level of artifacts. The latter is more decisive than the former... in my opinion. Indeed, for each map, many online players already know in advance whether they were lucky with their starting castle or not. That is, the rigid division of magic into schools and the binding of heroes to these schools is not a big plus. Experienced players often know that... here I will level up druids and summon Shriekers in the endgame, and here I will level up a different hero and use them to clear the map, but on this map, you can't pass without a tactician... and so on. It is difficult to criticize the magic school itself; I probably wouldn't change anything in it. But the monster system is simply terrible. Yes, exactly. Monsters of the first and second levels look more or less comparable, the difference between the second and third levels is already more noticeable, but the gap between the fourth and third levels is enormous. It's as if level 4 Shriekers are something extraordinary. This system has reduced the final battles to fights between heroes and level 4 monsters. Sometimes, very rarely, level 3 monsters are also used in battle.
In general, while writing this post, I came to the conclusion that I would change the monster stat system. I would bring them to a more or less equal level. More precisely, I would improve the first three levels so that they don't become so useless after 2 in-game weeks.