Thanks for all the comments! Please rate the game! Constructive criticism is welcome!
I know some things are not self-explanatory and just mentioned on the game's page in details.
The X signs are power up drop places, never drops where you are. You need to take the time between waves to find the packages (always two of them, can be weapons, health, speed upgrade).
There are three enemy types: normal zombies (lots of them), ghosts and some furry white masked quick and agile guys. Zombies are in every wave, other enemies are from wave 3. Each wave has more enemies and they are also getting stronger. There is no ending. You survive as long as you can. You get points for killing so for achieving higher scores, you need to survive more and more waves. The "?" weapon packages can be great or disastrous, but in overall, there is more possibility to get better weapon in later waves. High risk, high reward? Or epic fail?
As you can see, I kept older versions too.