Get the Big Guns
Get upgrades quickly to be able to use more powerful weapons, which are more effective in killing enemies. Upgrades appear when you kill enemies. They take the form of a black and white box. Walk over an upgrade to collect it. Acquire the upgrades quickly, though, because they disappear after a short time.
Make Effective Use of Barricades
Barricades are one of the most important items in the game. Laid out carefully, they can effectively separate you from immediate death. Block off zombie spawning points to keep them away from your character. You can create choke points by laying out barricades like a funnel from spawning points. This will herd all zombies into an increasingly small area as they chase you, making them vulnerable to gunfire.
Best Place for Turrets
Turrets must never be placed out in the open where zombies and other monsters can destroy them. Place all of your turrets safely behind barricades to make the most out of these automated zombie mowers.
Keep on Moving
Standing still at any moment in the game can leave you vulnerable to the swarms of zombies trying to engulf you. Once you are cornered, expect to lose a large chunk of life to get out. Always be on the move while shooting at zombies. Get into any open area to allow you room to maneuver around the enemies.
Careful with Explosives
Explosives such as barrels are effective in clearing out large groups of enemies, but they can inflict damage on you if you're close enough. Watch your health when making use of these weapons or you'll find yourself looking at the "Game Over" screen before you realize it.