The nation grew with the increase of population, the development of towns, and the growing mechanization of craft industries. There were watermills for crafts and for supplying and draining water in all parts of the nation. In flat areas, slow rivers could be supplemented by creating artificial waterfalls, for which water was raised to the level of reservoirs. There were also some iron- smelting furnaces. Coal mining underground began as a family enterprise. Stone bridges over rivers could accommodate one person traveling by foot or by horseback and were steep and narrow. The wheelbarrow came into use to cart materials for building castles and cathedrals. Merchants, who had come from the low end of the knightly class or high end of the villein class, settled around the open market areas, where main roads joined. They had plots narrow in frontage along the road and deep. Their shops faced the road, with living space behind or above their stores. Town buildings were typically part stone and part timber as a compromise between fire precautions and expense. Towns, as distinct from villages, had permanent markets. As towns grew, some became boroughs by paying a fee to obtain a charter for self-government from the king giving the town judicial and commercial freedom. They were literate enough to do accounts. So they did their own valuation of the sum due to the crown so as not to pay the sheriff any more than that. These various rights were typically expanded in future times, and the towns received authority to collect the sum due to the crown rather than the sheriff. This they did by obtaining a charter renting the town to the burgesses at a fee farm rent equal to the sum thus deducted from the amount due from the county. The freemen were "free of the borough", which meant they had exclusive rights and privileges with respect to it. Selling wholesale could take place only in a borough. Burgesses were free to marry. They were not subject to defense except of the borough. They were exempt from attendance at county and hundred courts. The king assessed a tallage [ad hoc tax] usually at ten per cent of property or income. In the boroughs, merchant and manufacturing guilds controlled prices and assured quality. The head officer of the guild usually controlled the borough, which excluded rival merchant guilds. A man might belong to more than one guild, e.g. one for his trade and another for religion. The frankpledge system prevailed in the boroughs.