A huge step up from Space Invaders in almost every way, and a great midpoint that would eventually lead to Galaga. The pressure isn't quite as constant as in the former, but it's made up for by the charm and diversity of its enemies and their far more diverse movement. Each color acts in different ways, and takes a more active role in your demise, swooping down erratically and throwing you into a frenzied panic. I have a lot of nostalgia for this, as I grew up playing it a TON, but it's still undeniably a super fun game and a great landmark for the genre.