A copper wall is a block which can be crafted from 2 copper bars. It has a mix of a light, mellowed greenish colour with a dark green hue. The copper wall is one of the blocks which can't be destroyed by dragons (below sand dragons) and is one of if not the cheapest blocks (based on general amounts of blocks found in a world) to make a shelter which is durable enough to withstand attacks from the majority of mobs.
The Copper wall can also be crafted back into a single copper bar, which is a potential loss of resources if a player has crafted a copper wall from 2 copper bars.
Copper is weak, so it's best used for short, practical use as shelters for the night to fend off the undead