i think the sheild upgrade is toooo cheap. YOu can freeze almost anything in there: such as a the last boss. ALSO. In the shield area, it will damage the object. it can destroy those pesky missiles those things shoot.
I find that the most versatile, reasonably cheap, and powerful, useful unit in the beginning is the knight. It is more useful than the gladiator by far. In the begining, you get the knight and spam it. It will get you pretty far.
Do you hate these pesky bullets those red ships shoot. Easy solution. Just buy damage. Once you max out your damage: lvl4 and lvl 3 power, you can easily wipe off any boss and enemy there is.
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