Battle for Wesnoth

That's where the real skill showed in the fourth mission; this is the beginning of great achievements in the future!

The main thing is not to let your upgraded units die.

If you want to play on easy, it's better to choose shorter maps. Also, some campaigns have a chronology. There are two dwarf campaigns, and that's exactly the case. First comes "The Fiery Scepter," and then "The Hammer of Something." The "Fiery Scepter" is actually quite interesting; it's epic. I also liked the last two (Northern Renaissance and Under a Blazing Sun), but you need some experience for those.