I have successfully reached level 7 or 8 or something. But, on the current level i'm on I can't proceed when I've killed the enemy. The "press SPACE to continue"-message appears, but when I press space nohing happens. What to do?

Exactly what level is this?
Did you try loading a savedgame from before you've killed the enemy?

This happened to me earlier and as always the answer (for me) was simple but took me forever to figure it out... The lord of the land had been subdued and put in prison rather than being killed so u might want to make sure u have the imprisonment turned off or just see if u have him in jail if so obviously just release him and kill him or torture to death if u have the time lol ... hope this helps