/topic/12565/
You'll learn quickly there.
Then move on to spiral campaigns:
/topic/14710/
After that, you won't be able to stop! I constantly spend several hours on my own campaigns.
Regarding saves. I recommend saving the game into separate slots. Here is my save file system:
at the beginning of the month (save "Month")
at the beginning of the week (save "1")
at the beginning of the day (save "2")
after every battle (save "3")
before every significant strategic decision (save "4")
after every captured town (saves "Town 2", "Town 3", "Town 4" etc.)
This way you will complete the map with minimal time loss.
When playing a campaign, you can make additional saves for each map, although it's not mandatory. I didn't need those saves at all.
Added after 2 minutes
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By the way, when I started playing, I didn't use magic at all (the "Book" icon in the bottom right corner of the battlefield). And it was fine; everything could be completed normally.