Yggdrasil is a Swedish studio with a strong house style — clean geometry, restrained colour and unusually good animation. It has consistently favoured original mechanics over licensed themes.
Valley of the Gods is the studio's best-known design: a locked grid that opens up cell by cell as symbols are destroyed, so the board you finish on is larger than the one you started with. MultiFly and 9K Yeti use multiplier ladders that build across a free-spins round rather than resetting each spin.
Yggdrasil also developed several of the shared mechanics other studios now license, including cross-game jackpot and tournament systems.
All Yggdrasil games on this site are free to play, with no download and no account. The grid-opening designs in particular reward a longer session, since the interesting board states only appear once a run gets going.