Stooner: I heard that's actually a tower offense. Is there really a big difference, aside from the difference between defending or offending?
The gameplay is vastly different. You get a limited amount of resources, and are supposed to build a convoy capable of traversing the maze-like environment. There are enemy towers all over the place, and you are encouraged to destroy them (gives you more resources that can be used to upgrade the vehicles in your convoy or buy new ones). While you can clearly see that they got the inspiration from tower defense games, in terms of actual gameplay, it and TD games are worlds apart.
I don't know of any other TDs on GOG. And let's be frank here, most TD games are not all that good. There are some shining examples of good TDs and a few alright ones out there though. Defense Grid is by far the best I've ever played, though Defenders Quest (which you said that you had just played) was also quite enjoyable. Sol Survivor could have been nice, had it been harder.