If only this game had missions and a story, the background could change with the location, stuff could be gradially released for use with mission+rank combination
For next game, which by law you have to make. You should add rewards for completing missions, perks of some kind, lower prices, more fire rate and anything really. Also some kind of currency used to boost performance of towers.
I'd like to see a sequel with larger meaning for trading, for example, buying/building/upgrading boats need certain materials to be in that town and power ups.
For those who have trouble with filling in the numbers after you've filled up the obvious hints, use the little boxes inside the boxes to mark which numbers are possible, the ones that aren't possible anymore will go away, thus leading you through to the solution.
Just realized, this game will make another great sequel, with player map pack + new mechanics and more harder levels, achievment bases unlocks and maybe even some kind of upgrade system, this has to happen!
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