It does seem like it could be a very good game for players who are into grinding. That's just unfortunately not me these days. Big picture, I agree with the other reviewers who (in general) said to let the players know even sooner that there will be a permanent upgrade shop after each game-over, and if it wouldn't be too detrimental to your overall design goals then I might suggest making newly unlocked structures like new towers or mana generators being buildable from the outset after getting the perma-upgrade instead of needing to also progress through a level enough to actually get that bonus card dealt during the level.
A couple other minor comments for the next round of updates: Making the camera pan around seems a little janky – two buttons at once to move diagonal won't work, and buttons won't work at all if the mouse is hovering over the gun upgrade window while you're pressing.
There seems to be a bug if you click twice at the same spot while building a gun without moving the mouse between clicks then I think it builds two guns at the exact same spot(?) but is definitely not building a gun and then opening the upgrade menu for it like I was expecting.