Element TD 2 is an artifact from the golden age of tower defense

Sometimes custom game modes and mods are transient experiences that come and go, flavors of the month. Some stick around though, or break out into their own standalone game or even genre. Games like DayZ, PUBG, or Auto Chess. The best-known example of a custom game becoming a standalone genre is the MOBA, growing as it did from Aeon of Strife for Starcraft, then Defense of the Ancients for Warcraft 3.