The GUI takes a long time to load and all the fancy grafics are mostly not needed (only relevant for shooting down spies). Also I agree with the comment about the game needing more balancing so iron and steel are not so attractive in the later game. Also look into giving the player a possiblity to see money/second and for upgrade it should be possible to see not the new total but how much it changes (e.g. +$100 instead of $3000).
There are many possible correct solutions to a puzzle, e.g. a square consisting of four numbers could have to correct solutions both 1,2/2,1 and 2,1/1,2. This is very annoying. Also the timer wraps around after 100 minutes.
 "Spidle Tridle"){kind=icon}
 "Mathdoku Game"){kind=icon}