For the love of all things good and holy, make spacebar an option for boost. X is in a horrible position and when using control, shockwave'll randomly lose focus, and control+w is the keyboard shortcut for closing a window.
As for a way to make defense viable, that's just going to be a bunch of number crunching. The easy answer is to lower the defense ramp, but that could easily throw things even more out of whack. Maybe a good first step would be to lower the ramp of the amount of damage the enemies deal, because once something gets through to the tower, you might have two to three more rounds and that's it.
Yes, it becomes more worthwhile at the higher levels, but think about how long that takes to get to for even one upgrade. Taking attack power, for instance, up to level 20 will cost 210 white blocks. To do the same with blue blocks, you'll have to earn around 200 brown blocks (which also goes to show it's foolish to put the blue blocks in anything other than the blocks per wave bonus, at least at first). I'd probably have them earned by damage dealt and have that amount ramp...
I've got to echo the comments of Vanderwaal and add to them. The balance is way out of whack. As is, it would take months of active play to get to any significant level purchasing the permanent upgrades, and frankly that's not going to happen because this is too active to be an idle game and too idle to be a more active tower defense. Incremental type games work because each trip through you have some impacting upgrade you can do almost every play through. The way this is balanced, that's almost true up to around the level 40-50 area, but then you just hit a brick wall. In addition to the behavior changes, I'd also rework the way/rate you earn brown blocks (what you trade for the blue blocks used to purchase upgrades). To put this in perspective, it's currently set to where you have to beat 10 bosses (I assume that's what triggers getting the brown blocks) which takes a few minutes to eliminate having to click for a 1 block purchase.
 "Earn to Die 2: Exodus"){kind=icon}
 "The Perfect Tower"){kind=icon}