Shield is a weapon from the first game. This is a Cross (十字架) in the Japanese version.
Looks like a heart in some versions of the game and is a fast weapon to fire but very limited range. It can destroy enemy projectiles as well as the enemies themselves.
In the first version of the Arcade game and the NES game, it is necessary to complete the 6th level.
In the C64 port, much contrary to the original one, the shield is a rather slow weapon, which goes through enemies after hitting them, hence allowing multiple enemies being killed by one shot. However, projectiles are not blocked.