The metacritic rating for this game is certainly correct. As someone that likes TDs, likes the art style, atmosphere, and music, I have trouble giving it more than a 6/10 simply due to all the gameplay ruining engine glitches. It probably doesn't even deserve that. Without the engine glitches, it might be a 7/10 or 8/10 TD.
There's a good assortment and variety of towers in PWD - nothing that leaps out as being incredibly unique, but they've got splash poison, single target poison, splash fire, single target fire, AA poison, etc.; what is unfortunate about the tower system is it comes down to luck whether you actually find all the tower cards or not. Even after playing through the entire game, then grinding another hundred levels and buying as many cards as possible in the store... I still did not unlock every tower. Bummer. Aside from that, some towers are slightly unbalanced. If you find some electric tower cards early and max upgrade them, building nothing but those towers and invis detectors is sufficient for beating all but ~3 levels.
I'm skillful enough at TDs that I had no trouble beating the game and collecting all achievements - which brings me to the next problem. The achievements unlock in-game, but some do not unlock on steam. There I am, in-game, and I can clearly see I've got an achievement from beating a level with certain requirements met (like holding onto a large amount of prime energy - used for tower building) - got it in-game, but not on steam. There's an elaborate reset procedure that you can do to give yourself a second chance to unlock such achievements, but if one of the two grindy achievements (win 500 games, build 10000 towers) locks itself down, you'll have no way to unlock them without the help of a tool like Steam Achievements Manager.
It's quite aggravating staring at a list of 134 achievements, 132 of them legitimately unlocked, and 2 of them frozen. You know that you can get (and deserve to get) the final two, but they are utterly frozen and unresponsive. If you do reset them and start from 0, they will likely freeze again at some point before reaching completion. Therefore, to get them you are forced to "cheat" (Steam Achievements Manager), all because of the company's incredibly shoddy programming.
Speaking of programming - this game is not epicly high quality like Defense Grid or even Defender Quest. A lot of the time projectiles or magic don't even register. I've landed an 8-wave wall of fire down on groups of enemies, then watched in bafflement as only the final wave caused damage, leading to leaks. For a few of the tougher levels, it can come down to luck on whether the damage you're causing actually registers. About a third of the time invis detectors don't even bother detecting invisible units, so for invis-heavy levels you have to layer quite a few of them all around or once again you may have leaks. (Or just use more electric towers.)
The game is missing some important hotkeys as well, like a sell button/key to rapidly sell towers. Instead you have to click on a tower, wait for an animation, then click a tiny sell button. It also lacks an 'Auto-Send-Next-Wave' toggle for those that prefer the challenge of waves coming as fast as possible. Instead you're expected to mash the 'Next wave' button, wearing out your keyboard or mouse.
Overall the game might be okay for a novice TD player, which doesn't get annoyed by engine bugs that are sure to ♥♥♥♥ off a veteran - but if you watched TotalBiscut attempt to play it, then you'll realize the difficulty curve is a tad too steep for novices. Quite unfortunate, as that leaves a very small audience that will fully appreciate the game.
The paper voice acting is also far below the standards of much older TDs like Defense Grid. Overall it left me uncaring about the storyline.
5/10 or 6/10 seem like fair ratings for the game, until they patch some of the engine glitches. But the company does not seem interested in doing so at this time. Every time I see a changelog I get hopeful, but then I check and everything is still just as broken.