Yeah sorry if the in-game instructions were too cryptic lol.
To expand on this in the zombie bit:
- Mines work the same as vines, and will explode when zombie are close
- Turrets work the same as walls but you can walk through them
- Either of the buildings can be placed on top of dead vines
- Click to shoot at your mouse pointer
- Press left shift to upgrade your gun, which increases it's damage and attack speed