The BUZZAR is the best weapon by far, plus it and its ammo are cheap. The ZF-1, Plasmagun, and Rail-Lightning gun are not as good and way too overpriced. You should fix the bug that takes away your skill point if you go over 10. Also, make a better plot.
All levels on all difficulties are easily winnable with the same startegy: first make 2 additional miners, then make 1 swordsman and control him to kill off the first couple of attackers, once you have 400 gold make an archer and set the command to attack mode. Keep making archers till you have about 10, and then start spamming with swordsman. The archers will kill off any attackers while the swordsman destroy the statue. This should work for every level as long as you fully upgrade the archer and miner, then upgrade swordsman and statue gold production.
If you want backstory you better play the last 5 games...
I thought this submachine was pretty weak compared to past ones.
The last secret room (no spoiler here, but they're all really easy if you don't rush through the game) is definitely hinting at submachine 7.
Great game, story was pretty good (a little too predictable about the double cross). Another thing is that there's really no need for the freebie DDOS in each chapter, the story mode levels are easy enough. Nice job with the challenge levels, the final bonus is hilarious. Overall a great game, keep making 'em.