Blockade is at the origin of all the surround-type games which will follow, including popular lightcycle game inspired by movie Tron. Even famous snake games owe everything to Blockade.
The principle is simple, you control a kind of snake (in fact a growing brick wall with an arrow at the end) and must last longer than your opponent on the playfield. You die if you crash into another wall (yours, your opponent's or walls surrounding the area). The best tactic is of course to try to surround your opponent so that he panics and gets trapped within the walls.
It's 2 player only game. You can't play against the computer. Even if it looks primtive now, Blockade was a real milestone in gaming history, creating a whole new genre of its own.
This was Gremlin's first video game. It provided Gremlin with a big hit, winning the 'Best of Show' award at the 1976 MOA convention.