Great stuff - I like it. But it's a little difficult for a start. Maybe you could start with fewer enemies at the beginning (maybe just one for a while), and then it could get harder later? Maybe it's just me, but I tend to not survive very long in this...

It runs very nicely (fast and smooth) in my browser. It may be that your browser Java plugin is an older Java version than your Greenfoot Java version. Try to check and upgrade if that's the case.

what are the structures in the middle (the rectangular brown thingies with the twiddly blue thing on top and the graveyard next to them...)?