Thanks for all the suggestions. They were very helpfull. I've managed to fix few bugs and added some new stuff.
- score bug fixed: now it is current level * 10000 + score at level;
- when kill enemy, a score value appears at his position;
- when level changes, a green info appears "Level Up);
- when a berserk mode is available, red message appears at players position;
- modified background animation;
- added new screen: Instructions where everything about game controll is explained;
- levels switch faster: after 1000 points;
- new resolution is set to 800x600;
 "Triangle Scavenger"){kind=icon}