Da Vinci's Vault Slot by Playtech

The free spins have no set length. A Cryptex landing opens the Vault round, and instead of counting down a fixed number of spins, it runs until three Skull symbols turn up. Until that third skull lands, the reels just keep going, and normal symbols convert into wilds along the way, so a hot run can snowball into something ugly for the house. It's an old idea done well: the round might end on spin two or spin fifty, and there's no telling which.

The base game leans on two symbols. That gold WILD shield with the green gem substitutes for everything except the Cryptex and the Compass Coin, and it ties the Mona Lisa for the top pay, five of either landing 10,000 times the line bet. The Compass Coin is a scatter that can breed: land one and it may throw 2 to 5 extra coins across the reels before the win settles, padding the scatter pay and nudging the feature odds upward. If you'd rather force things, the purple Power Play button switches on a higher-stakes bet mode that lifts your chance of a big hit or a vault trigger. There's a wheel tucked in as well.

The reels sit inside a candle-lit gothic cathedral, stone columns and stained glass behind them, red velvet drapes down each side, four huge golden gears turning in the corners. Royals come drawn as ink sketches with paint splashes flicked over them, while the premium icons run through Leonardo's greatest hits: the winged flying machine, the Holy Grail glowing in a blue orb, the Vitruvian Man boxed in his circle. It's a 2017 build and plays like one, no tumbles or cascading multipliers, closer to a straight lines game dressed up as a treasure hunt.