The trick isn't to buy a ton of turrets and spread them out.
For the first dozen or two waves, you should only have 2 turrets, all maxed out.
From then on, you should be focusing on building 1 turret at a time, and maxing it out.
Also, spreading them out is dumb. Putting 3 on corners is great, since they shoot fastest at 90 and 45 degree angles, and a large accumulation of them in a single spot will wear out the enemy faster for when they hit the next set of turrets.
If you can get 2 or 3 turrets on each corner, with a good deal of upgrades on each, you will get a long way without having to do much.
Also, start out with only 1 water tower. They are good because they have a better damage increase for each upgrade. However, don't waste the extra $1000 on the last upgrade. Instead, keep that money for all of the seed-towers, as the jump to it's final stage is something like 9,000 damage.
Use the water tower to get the money for the first few seed towers, then buy and upgrade nothing but seed towers afterwards.
The best way seems to be to max out one water tower, it can last by itself to around wave 30-40, and then place one maxed seed tower in a row at spots where it can shoot up/down or left/right, so you get them firing at their fastest rate for the most amount of time. Also think about where you place them with respect to how far they can fire, thats why we mostly place them in the middle areas.
You dont even need a water tower. You have enough money in the start to build two seed towers and up 1 one time. That is plenty to fend off the first few waves, and slowly upgrade them. That will save you the money of the water tower.
One seed tower can fend them off if you place it right. Water tower is just easier to begin with. You can also just upgrade a water tower to like 4/5 level and then work on a seed tower, save even more money.
wow, this game isn't even remotely fun, i was expecing that one where you throw the people around and keep them from killing your castle. Big letdown...