Probably, that common origin point are the Kingdom Rush games. (All screenshots taken from Kingdom Rush Frontiers, because Kingdom Rush is a pain in the butt in terms of difficulty. Well, they both are, but Frontiers is less of one.)
Like Incursion and its sequel, the Kingdom Rush games have garrisons that spawn and endlessly respawn soldiers,
And herein lies the flaw that keeps me from liking the games as much, even without the fact that Incursion feels better balanced: While Kingdom Rush has a few genuinely entertaining towers, such as the "DWAARP," an earthquake-generating tower...
...its infantry are usually boring but necessary.** (In the above environment, for instance, there are enemies who are immune to attack until they've been intercepted by infantry, which is a jerk move when one is in a situation where infantry are hard to keep alive, such as the desert environment featuring the same enemy and random emergences by a giant sandworm that instantly devours everything it attacks, friend or foe.)
The game also has heroes and spells, and I will admit that the systems of spells and heroes Kingdom Rush use are better in many respects than Incursion's; Incursion's spells and Incursion 2's heroes both have certain flaws, that being random acquisition of spells and being a bit of a pain to micromanage, respectively, while Kingdom Rush's spells are consistent and reliable (being on a simple cooldown timer instead of being random, infrequent enemy drops) and its heroes don't actually need micromanagement (and can't actually be micromanaged, for that matter, which for me is largely another plus).
On the other hand, Kingdom Rush's actual spells are terrible. (Its heroes also aren't terribly intuitive; the game defaults their positions to in front of one's gate, but they really ought to be at either an enemy crossroads so that they have the chance to attack as many enemies as possible, or, if there's only one enemy entrance, as close to that as they dare. They die a lot more, but they have to be close in to get the experience they need to level up. Incursion 2's heroes, on the other hand, can be useful no matter where you put them.)
Well, no, they aren't that bad, but they're too concerned with being "balanced" to be very good. One of them drops "reinforcements," a pair of little soldiers who vanish after half a minute or so, who are handy for blocking enemies in desperate situations... and need to be upgraded a lot if you want them to have enough health to actually be useful after the first few missions. The other is a slow-striking area-of-effect spell that does some pretty decent damage... and actually hitting anything with it takes a lot of work. (It actually seems stronger in the first game, but they changed it for the second, and it feels a lot weaker.)
That's not to be all down on the games. While I like Incursion better for aforementioned gameplay reasons, Kingdom Rush is a teense more polished and has a bit more humor. For instance:
The guy who is more or less the main villain seems to have a vocabulary made up almost entirely of jokes and referential quotes. If one looks at the above desert level, there are also some things that anyone with pop cultural awareness from the last thirty-some years ought to recognize, for that matter.
These games do have a lot of cool things in them, and if one likes Incursion, one is likely to like Kingdom Rush just as much or more. Which one likes best is dependent on personal taste.
*Part of my relative dislike of Kingdom Rush is that many of these features feel much more like jerk moves; for instance, there are several maps, including one of the above, where new enemy paths just appear without prior warning, and usually in positions that let them bypass most of one's defenses. Kingdom Rush Frontiers in particular also has random "debris" cluttering up the build points in later maps so that you have to pay extra money to build your towers. On the other hand, while they're also harder to beat, Kingdom Rush bosses are also generally more entertaining.
**The first game's Barbarians manage to be pretty unique and interesting, actually, which is surprising, because the first game is usually a bit more generic.