I'd like to point out that, after going through all possible upgrades (or as close as I got to finding all of them through resetting then exploring each thread with 27000 embers) that regeneration, multicaster (and its connected upgrades), and possibly critical strike are the only ones that seem worth investing in at this point. At least that will give me three good options to choose from at the outset of a new game, and I can choose regeneration to start and the others later. While strategy might be part of any good game, in a tower defense game, it seems just a little out of place to be able to buy so many different upgrades that might interfere with getting useful upgrades, making the game LESS playable and LESS enjoyable.
Not trying to sound mean or ungrateful. I'm still playing because I see a way to do better, get further, and gain the sense of accomplishment playing video games gives me :-)