Lozenge-shaped siege money minted in Newark-on-Trent, 1646

Money minted during a siege.

https://en.wikipedia.org/wiki/Siege_money

In the early modern period, it was necessary in Europe because at that time coinage had an intrinsic value and even though a town was besieged commerce had to continue and soldiers had to be paid.

Not sure as to why the shape, might be ease of manufacture or somesuch.