Infinitode 2

I haven't played a lot of tower defense games in the recent years. They don't seem to be very popular games to make, contrasting with the Flash game era, where a new one was popular quite often. That said, Infinitode 2 is probably the best tower defense game I've played, as far as pure tower defense games go.

To address a possible initial concern, Infinitode 2 is free, and also available on mobile. It sustains itself off of microtransactions. Basically pay-to-progress-faster. There are no paywalls, but because the progression possibilities are nearly infinite, paying does give you an edge. But I wouldn't care about that one bit. There is of course no PvP in the game, and only your leaderboard ranking is at stake. Especially on PC, I didn't feel any incentive to pay a cent.

Now, Infinitode isn't super innovative, but it does have a good amount of depth. Various enemies walk along a predetermined path. You place your turrets outside that path. Enemies have different properties and are resistant or vulnerable to different towers, promoting tower diversity. You'll also want to diverisfy your towers on account of which enemies take which path, dissonances like ice and fire not working well together, or just general placement ideas like spreading out your freeze towers to cover, but not overlap, the entire area, or placing venom towers near the start, to ensure maximum poison duration.
The levels start off simple, but later on, figuring out the optimal tower placement (or just a kind-of-good placement) is like a puzzle that needs to be solved. Problem is, it's work you should ideally do before starting the level, and not something you would need to re-do in the future, unless you noticed an ineffiency in your placement. But because the game has a lot of vertical progression in the form of an extensive upgrade tree, you will want to re-visit levels again and again to get a better score, and farm more materials. Upon re-visiting, however, you will have to remember how you wanted to place your towers, which I found to be an incredibly tedious task from the second time onward.

Overall, despite putting over 40 hours into it, I find it somewhat hard to recommend. The long playtime comes from the grindiness of it, and while unlocking each new turret did give new interesting options, as did new levels provide new challenges, it didn't feel rewarding. Again, because of the highly vertical upgrade progression, the difficulty in levels (most of which are infinite, as far as I know) had to raise rapidly as the waves went on. The early waves were trivial, meaning you could get through them pretty much no matter what you did. Then there were a few waves which were matched to your current progress, where your success depended on how well you had designed your defenses. And then you got overwhelmed, regardless of what you did. My actual skill had a marginal effect, and my performance was mostly dependent on how much time I had invested into playing.
Still, I had a decent amount of fun for about 30 of those hours, and since the game's free, I'd totally recommend giving it a try until you get bored.