Zeus 3 is the first WMS slot I have added to GSM's free-play slot machines but it's a cracking game IMO with a good balance between big wins and extended game-play. I'd call it medium volatility but the whole thing is just pretty entertaining with it's expanding wilds and original bonus game.
The feature is triggered by 3 or more scatters (although Zeus 3 has 6 reels, only the last 5 reels have scatter symbols) and you get 10, 25 or 100 free games depending on whether you get 3, 4 or 5 scatters - just like the original Zeus slot. At this point, the reels flip round so the reel with 7 symbols is to the left. Additionally, much like with their Colossal Reels games like Spartacus and Giant's Gold, any wild symbol that appears during the free spins on any reel expands to fill the reel.
You won't see this version of Zeus 3 in European land-based casinos generally, mainly in North America. For some reason, there is one of those wierd 3D-style versions for Europe much like the Wizard Of Oz Wicked Riches game and IMO the version above is way better. But then again, Europeans are more focused on Novomatic slots like Book Of Ra and the popular Sizzling Hot slot so they probably won't even notice ;)