Great Game! Just beat all levels. Got stuck for a while on quite a few. Here is a tip for the higher levels. Use multiple physics labs to create an unstoppable swarm. Physics labs with supper carriers will decimate almost anything. Ships are almost always a better investment than turrets because they will keep coming back. On hard defense missions don't kill the first wave, you can get all the money you need before killing it. But yah, super carrier plus 5 physics labs is unstoppable.
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