**Blockers** are units, whose only goal is to have a very big armor. In earlier rounds of Planetech blockers were used in battles as a **shield** for the rest of the fleet. Units without weapons were relatively inexpensive to produce, so it was easy to block whole battles with those cheap units.
Units that do not have enough firepower are specified as **Blocker** while designing them. If blockers take part in a battle, those units are always attacked at the end, after every non-blocker has been destroyed. Like this it is not possible to use blockers as a shield for other more expensive units.