It just takes too long, but a great risk variant.
The buildings should be exchangeable, but for a cost, perhaps each one should cost 100gold regardless.
Strategy
Start with a machine gun turret, then build an Electro Tower. Build machine guns and electro. When you can afford it the Microwave tower is great, and the ice tower is good for slowing down when you get a trap.
Get your Mercenary level as high as you can, then Grenadier and Engineer should be second or third. Marksman is great, but not necessary until the later levels when the spawns/trails are further apart. Ignore Scientist.
If you get stuck, go back to an earlier level you can beat easily, and do it again and again to get skill points, the higher your score, the more points you get.
It's a good game, but sometimes it's quite difficult until you have your skills to a certain level.
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