This is a slightly older game now but it's still one of the most fun flash games around. It also involves in-depth strategy not often found in flash games beyond the tower defence genre. You play a special forces operative who has holed himself up in a facility and must fend off the zombie hoards.
In order to fend them off you need to understand the balance between exploration and defence right away. The more you explore, the more areas you open up and the more windows, doors, and wide open spaces the zombies have available through which to climb in and get you. Zones which you have not explored are blanked out, and zombies do not bother trying to get through there. The F key will upgrade your barricades and space will take you to your inventory where you can buy weapons.
You start off with a pistol, but you can unlock a free rifle by signing up for a free Mochimedia account and buying the free rifle in the premium tab (other special upgrades cost mochi-coins). You can also claim the free grenades to get you started. The rifle will serve you well until you're out of your first area. The best strategy is to fend zombies off the first area until you have about 300 dollars of in game funds, then leave the area and immediately pick up the Uzi sub machine gun unlock. Then press space and go over to the inventory screen to buy the sub-machine gun you just unlocked. This Uzi will serve you well, and just as well because you now have a larger area to defend.
Make sure you keep your barricades up and bottleneck the zombies at the door to the first area until you've killed enough zombies and gained another 250 dollars in game. Then run down and unlock the car porch. This is not going to effect the number of zombies coming in because the lower areas are open anyway, but it will give you access to an AK47 and give you more places to run should everything go pear-shaped.
I do not recommend buying the AK 47 as it does not offer significant improvement over the Uzi, but will set you 300 dollars less towards your next upgrade (one that matters). Do not upgrade the barricades either, as these upgrades can be used strategically later on. At this point, the best thing to do is to defend yourself with the Uzi, using the available terrain to your advantage, until you can reach about 1600 then unlock the central door that takes you into the canteen and then down into the kitchens, there you will find the SCAR Assault Fifel for 1250 or a G3 Assault rifle for 1500. Both are very good, but the G3 has the edge due to its stopping power. Zombies will physically stop when you blast them with it giving you more time. The SCAR has some plus points though, its rounds pass right through zombies allowing you to finish off several zombies at once. It is however better to go for only one of these and save up your remaining funds towards your first turrets.
The turrets are in the small room that needs to be unlocked in the lower left of the map. Once you have around 600 to unlock that door it's a good idea to go and do so and pick up the turrets to unlock them. Later, once you have the funds (4,000) you can start placing your turrets. Turrets will eventually run out of ammunition, but you can reload them by pressing F, just in the same way as you fix the barricades. Because of this, turrets are a great investment.
There are two methods to using your turrets. You can either cluster your turrets in a room so that you will be able to easily access them to reload them. Many people instead chose to reinforce all the barricades by placing turrets behind them to kill zombies before they even reach them.
Which tactic is up to you, but having played and replayed, the clustering tactic is the one that seems to work the best. When you cluster your turrets they are all accessible enough that you can run around them and reload them. If you have chosen to shore up all the barricades, there will be times when you're unable to reach certain turrets to reload them and they are sitting there doing nothing. The clustering approach certainly helps to ensure that you have all of your firepower turned on the zombies at any given time.
Once you reach this stage, all that remains is to fend off the zombies until you hit the final wave. Good luck.