Apparently, even historians of Indian history are unaware that in the nineteenth century, a huge hedge, over 2300 miles at its longest, traversed much of India, built so the British could rake in even more cash from even the poorest Indians via a Salt Tax (and to a lesser degree, a tax on sugar). Take a look at Roy Maxham’s 2001 The Great Hedge of India (several editons, I think) for details.
Scarce commodities or those with relatively few sources make good targets for control and taxation (good economic basis for political powergrabs). Salt can only be mined from or made in certain places (including along sea coasts), so the British were able to command construction of a barrier to trade into British India made of thorny plants, sometimes living (think acacias, prickly pear cacti, and euphorbias), or piled branches (harder to maintain but possible when plants wouldn’t take or were killed by vermin), with portals 
every 4 miles or so manned by soldiers and customs agents. They abandoned the hedge in 1879, and its grade was reused for road beds (especially where it was elevated), or retaken for fields by farmers of adjacent plots. The hedge permitted collection of a tax on salt so high that made its cost prohibitive to most of India’s populace. At the same time, they taxed sugar passing the other direction.
Smuggling was common. Bags of sugar were loaded on camels that were rushed through the hedge. Others were lobbed over the hedge for collection by accomplices. Nevertheless, the British obtained a huge profit despite needing over 12,000 men to maintain and police the hedge.
Tidbits: Timbucktoo flourished because it controlled salt trade; salt tribute was taken in China as early as 2200 BC. Want more? Read the original….