Mod for Heroes II.

Monster parameters, as well as building costs, are stored in the executable file within data structures. It's easy to find the monsters: Open the executable file (e.g., HEROES2W.EXE) with a hex editor and search for a string, for example, "drgn". The first occurrence will lead you to the middle of the green dragon's data structure. All monsters are arranged in the order of the castles. The first are peasants, and the last are water elementals. The structure stores the main parameters, cost, attack and defense values, growth rate, etc. There is also a special byte that controls additional parameters, such as whether a dragon can fly and occupies two cells. In this byte, you need to look at the first four bits (or something like that), which allow you to adjust 4 parameters. I don't know how to make a monster immune to magic like a dragon; I haven't found that parameter. You can also find, for example, the cost of buildings. These structures are located around address F267C (specifically, the cost of the pyramid for the barbarians is 6000 = 1770 in hex; other related values, such as the additional 20 crystals, are located nearby). I didn't go any further because I was only writing an editor for monsters.

Added 2 hours 24 minutes later
And for dessert. I just found the cost of the Summon Bolt spell in the executable. $-) F1888 - this is where the structure of bytes like 050A0A0A is located. So, 05 is its cost. The other spells are located nearby. What the other bytes mean is unknown; I didn't investigate.