It's the same engine as SS... and same style gameplay. They tried to add in a few more things to make it interesting. You launch yourself on a catapult, things pop/crawl out of the ground instead of just appearing. Also you collect treasure that lets you buy(Costs 50 gold) powerups at an alter. You can use these latter buy hitting the key bound to it. Also killing different monsters with different weps has affects. So it's not always about using the most powerful wep. Some of them you shoot and they gib and then the gibs morph into litte more annoying versions of the monster you just killed(2 usally) So if you use, say the flame or acid wep they will burn/pop and not turn into the littler guys. Then again you've got 50 of these things coming at you and those weps fire insanely slow.
This demo is okay. Dosn't make me want to buy it though. I played through both the SS games, and even with these new things it dosn't seem like enough of a step. The weapon switching is NOT FAST ENOUGH for this type of game. If you use just next/prev wep to switch, instead of the number keys, you will be stuck waiting for your char to pull out the wep while your mowed down by the wave of guys. And you NEED to switch, because of said poping guys and having to kill things with different weps. Also it started getting annoying because you can't save anywhere.
Sadly these little things just get too damn annoying at times.