Hello. This is a classical tower defense game.
You face various enemies coming in waves.
The enemies run from a starting point to an end point along a specific path.
You can kill those enemies placing on the cells nearest to this path some defenders (left-click on the cell and choose the defender you want to build).
That's all. Your objective is to survive.
Remember :

• Every killed enemy gives you resources to build more defenders
• Every end of the waves, if you are still alive, you earn other resources
• Every enemy that cross entirely the path subtract some life from you, until you die