Well, it's Space Invaders, we can't ignore that.
I remember the Atari cartridge with over 100 variations on the simple idea of Space invaders, so I know there are a number of different ways to do the game. This one still brings in something new, though. Let's assume you know the premise, last tank defending Earth from invading Titans, etc.
The aliens are varied, both in their looks, toughness and attack patterns, and the end bosses are screen-filling, multi-target monsters. This gives an old-school arcade feel for "Learning the level" Meanwhile you will be trying to earn money to upgrade your weapons, shields, etc in the hope that you will gain an edge over the enemy.
The looks are divisive. Very retro graphically, with screen warps (that can be turned off) to really evoke the 8-bit days. The price is very reasonable, especially when I consider what I would have paid coin by coin playing this in the arcades.