Purrrminator

Purrrminator by Relaxgaming is a video slot with a playful mash-up of robotic cats vs street dogs, set in a gritty urban alley style. The game is built on a 6-reel, 4-row grid in the base game, expanding to 6×5 during free spins, and it uses a Connected Ways mechanic instead of traditional paylines to reward clusters of matching symbols on adjacent reels. The core mechanic, called Reverse Avalanche, locks in any winning symbols, removes all non-win symbols, then causes new ones to drop in — continuing the process until no more matching symbols land. There are Expanding Symbols too, which can grow up or down a reel when they appear after avalanches. Free Spins are triggered by landing 3 or more Bonus (scatter) symbols; depending on how many Bonus symbols appear, the player picks a number of free spins. During Free Spins the grid expands and all base game mechanics, including Reverse Avalanche and Expanding Symbols, remain active. Players also have the option to use a Feature Buy, which allows them to immediately enter the Free Spins round for a fixed cost equal to 50× their bet. The maximum win potential in Purrrminator is capped at 5,000× the stake.

You can play it for real money.