This is a classic tower defense game. It has 3 maps and each one has 35 levels. You have to beat all levels of each map to unlock the next one.
The rules are like other tower defense games. You have to prevent enemies (which are all ghosts!!!) from passing through roads and kill them all before they exit the screen. You have 5 different towers each with a unique feature and have to use them wisely if you want to pass all levels!
As you progress, ghosts become faster and stronger and so the game will become more difficult. Good luck in defeating all of these ghosts!