Building Book I: 300 levels, one idea at a time
Ten chapters, thirty levels each, and a lesson from Candy Crush about teaching.
Book I of Crystalbound Saga is ten chapters of thirty levels, three hundred in all. Every level is a fight: you match elemental tiles to hurt a foe, and the foe hits back on a timer.
A weakness cycle
There are five elements: Ember, Tide, Storm, Loam and Dawn. Ember beats Loam, Tide beats Ember, Storm beats Tide and Loam beats Storm. Hit a weakness for double damage. Dawn is special, because it is the only thing the Hollow are weak to, and Dawn tiles also heal.
Teaching one thing at a time
The first five levels used to introduce around fifteen ideas, which is far too many. Candy Crush teaches roughly one new idea every ten levels. Chapter 1 now works the same way: basics and special tiles first, then Chains, then Miasma, then Relics, then Elemental Wards, with a mini-boss every fifth level.
Three Guardians
Mirelle, Rosalind and Verdanne are your companions. Fill a Guardian's gauge from their element and you can cast their Art. The story decides who can fight in each chapter, so you meet them gradually.
Tuning it by playing it a lot
To check the difficulty, I ran a bot through all 300 levels many times on each setting. It won about 96% of the time on Hero, 98% on Story and 82% on Legend. Levels that came in under 70% got extra moves or less boss health, so now every level can be beaten.