As villa manufacture was confined to articles of common need, more elaborate tastes had to be gratified by importation from places beyond the limits of the northern countries of Europe. The Emperor gave great attention to guarding the frontiers, so that foreign commerce could be carried on in security. The great trade routes followed the rivers. There was a regularly developed system of markets and fairs held near the cities and the monasteries, as in the case of St. Denis, near Paris, where for a space of four weeks goods were exposed for sale by traders from Spain, Southern France, and Lombardy. In Germany and in the more remote portions of the Empire, near the Slavic frontiers, the government established shelters and exchange offices for the convenience of merchants, and strict care was taken that arms were not sold to the enemy. Chief among the entrepots of commerce was the city of Mainz, famous for its cloth manufacture. Charles planned to make of it the great imperial economic center, and in pursuance of this program provided for the construction of a wooden bridge over the Rhine. He proposed also to build a canal to connect the Danube with the Rhine. But the bridge was destroyed by fire, and the canal offered too serious difficulties for the engineers of his age to surmount. Trade between the Empire and Great Britain and Ireland was encouraged. There was a lighthouse at Boulogne, and at Quentovia, now Étaples, a customs-house was established and placed under the supervision of Gerrold, a shrewd man of affairs, abbot of St. Wandrile. The constant stream of pilgrims passing from the islands was protected by the Emperor, and they proved useful in drawing closer the commercial ties with these remoter portions of the civilized world. Naturally the Mediterranean commerce was the more important, and the Emperor was careful to keep up good relations with Eastern princes, both Christian and Moslem. Imports consisted of purple stuffs, silk cloaks of various colors, worked leather, perfumes, unguents, and medicinal plants, spices, Indian pearls, Egyptian papyrus, and even exotic animals, such as monkeys and elephants. The cities in Southern France were especially frequented for trade, many of them having a cosmopolitan population. The Jews were valued for their business capacity, and also for their knowledge of languages and medical science. They were not allowed to own landed property, but no restrictions were placed on their loan operations, or on their commercial ventures. A marked improvement is noted in the coinage. After 800 the bust of the Emperor appears with an indication of the Roman military cloak and the words “Carolus Imperator”; on the reverse is a temple with a cross and the inscription “Religio Christiana.”