The ubiquitous concrete block, commonly called a cinder block, is one of the many tools of modern construction. They combine many of the best features of bricks and stones. They’re relatively easy to construct, too. Just pour concrete into a mold, let it set, and you’re done. Sears used to sell such molds to consumers in the 19th century. All you need is the ability to make a mold that is durable enough to be used repeatedly. Could all of this have been developed earlier? Of course, given that we’re discussing concrete, the obvious candidate is Rome. And the Romans were no strangers to such methods. I’m imagining the Romans building some impressive fortifications with concrete blocks, filled with packed earth or sand for strength. Not to mention all the handy things they could with piping in the voids of the blocks for civilian construction.