Defeat boss in top-down hack-n-slash style combat with lots of abilities.
Farm materials needed for the unlocks of the boss.
Use unlocks to upgrade base/castle.
Use upgraded base/castle to upgrade gear.
Find bigger/better boss to defeat, and go to step 1.
After every 3 or 4 bosses they introduce a new gameplay mechanic. More abilities, more dangers, potions, etc.
Hence, you are always unlocking something that feels like progress.
The castle building is also done nicely. You grow if from a couple of wooden palisades and a wooden coffin, to a multi layered castle with different rooms (for crafting bonuses), farming areas, a dungeon, servants that you can send on raids, and a lot of decoration options.