Every time I play Byakuren's stage, I feel like I'm playing Space Invader >_>
But seriously, ZUN himself had said that Touhou is one big fangame series. One of the reasons he thinks Touhou should stay strictly on the realm of doujin. Many of the songs, characters, and other aspects of the game are influenced or outright taken from other sources. Despite me saying that, there are very few things I find similar to other stuff. I have zero ability to recognize that stuff.