I like those that put a different spin on it:
Space Run: Fast and Safe Delivery: where your spaceship is your fort. Available on GOG.
The
<span class="bold">Creeper World</span> series, combine Tower Defense with Offense.
Available DRM-free from the developer.
Particle Fleet as well, from the same developer and website as above.
[url=https://indiegamestand.com/store/675/sbx-invasion-base-defense/ ]SBX: Invasion Base Defense [/url].
An arcade space shooter combined with Tower Defense. DRM-free on IGS.
Infinum Strike is available on GOG. I really like the game. There's a demo on GOG for you to try.
AI War: Fleet Command must be mentioned. Protect yourself against a swarm of AI. Is on GOG.
Unstoppable Gorg was very cool.
Available DRM-free on IGS.
Infested Planet plays similarly to a Tower Defense game, it actually reminds me of the Creeper World and Particle Fleet games. It is available on GOG.
Sang-Froid: Tales of Werewolves puts another spin on the genre, humorous and very unique.
Available DRM-free on Humble Store,
as well as on GOG*.
* post-edit