Sandbags

Sandbags are very interesting and underrated, as they have lots of uses, mainly defense purposes.

Main use
Sandbags are super cheap, only 300 energy, and have 250 HP, which is more than half the HP of Armour. They don't use anything to be repaired, and their build cost means nothing (unless you need to repair) as they start tanking damage as soon as they are placed.

Sandbags are generally (and almost exclusively) used to protect Turbines, as they don't block wind.

They are sometimes used to defend the roof of forts, or light Weapons placed on the roof (Machine Gunners, Sniper...)

Other use
Sandbags are one of the best defense against Firebeams, as they completely block its beam, preventing Fire damage.

They can be placed on the roof of your fort to defend against T3, because of their high HP.

One player has been known to be able to T3 sandbags (making them vertical), to defend against Firebeams. However, I don't find it very effective nor fast enough to do. Instead, it is advised to place several layers of wooden boxes with sandbags in them to cheaply tank cannon fire and firebeams.