As the Boer attacks on the railways became more numerous, something cheaper and more quickly and easily erected was needed. Major Spring Rice of the Royal Engineers, then in Middelburg, devised a blockhouse made of two concentric cylinders of corrugated iron filled with shingle. These were proof at least against rifle bullets and cost but £16. A party of six sappers with some African assistants could complete a blockhouse, its ditch and wire entanglement in six hours. Construction crews could build three blockhouses per day.