Those of you that have played at land based casinos will have more than likely come across Griffin's Gate in your time. It's part of the Hot Hot Super Respin series of slots from WMS, which includes the slot Plataea and Tiger's Realm. All three games and several more from the WMS slot range have been released online, however online gaming laws prevent them being played in the US currently.
As well as having the Hot Hot Super Respin, Griffin's Gate also has stacked door symbols, which reveal matching symbols when they open. This can particularly rewarding if you get several appear across the screen to form winning lines.
The bonus round in Griffin's Gate is triggered by getting 3 Griffin scatters on reels 1, 2 and 3. These are the only reels that this symbol appears on. If you achieve this you will get 10 free spins, with the added bonus of having extra wilds and stacked door symbols appearing. This gives you a greater chance of triggering the Hot Hot Super Respin feature. You need four stacked door symbols on reel 1 to achieve this. The reels will then respin 3 times, holding any wilds or stacked door symbols in place. As you can imagine, this can lead to some very big wins if the right symbols fall into place.
Griffin's Gate is a very entertaining WMS video slot and I have been known to play it for hours in the casino. I hope, like most US players, that the laws are changed soon so that I can play it for real money online - that's wishful thinking.