As a result of Roman conquest, many signs of Roman civilization spread over Britain. There had been no cities in Britain before the Romans conquered it. The civilized Romans were city dwellers, and as soon as they had conquered Britain they began to built towns, splendid villas, public baths as in Rome itself. York, Gloucester, Lincoln and London became the chief Roman towns; there were also about fifty other smaller towns. London which had been a small trading settlement before the conquest now became a center for trade both by road and river. Colchester, Gloucester, York and Lincoln sprang up round the Roman military camps. The town of Bath became famous for its hot springs. The towns grew up as markets and centers of administration. In most towns there were market-places and plenty where merchants sold their goods. The rich merchants and officials had luxurious houses which contained many rooms, with mosaic floors and central heating. Every Roman town had a drainage system and a good supply of pure water. Temples and public baths could be found in most towns. The Roman towns were military stations surrounded by walls for defence which were guarded by the Roman warriors. The Romans were great road-makers and now a network of roads connected all parts of the country. One of the chief road was Watling Street which ran from Dover to London, then to Chester and into Wales. Along the roads new towns and villages sprang up. Great tracts of forest were cleared, swamps were drained, and corn-fields took their place. The province of Britain became one of the granaries of the Roman Empire. A constant trade was carried on with other parts of the empire. The chief exports were corn, lead, tin, and building tiles. The goods were sent in wagons along the roads of Britain, Gaul and Italy to Rome. Britain imported luxury goods, especially fine pottery and metalware. But together with a high civilization the Romans brought exploitation and slavery to the British Isles. Rich Romans had villas in the country with large estates, which were worked by gangs of slaves. Prisoners of war were sent to the slave-market in the Roman Empire. The free Celts were not turned into slaves but they had to pay heavy taxes to the conquerors and were made to work for them. The Romans made them clean forest, drain swamps, built roads, bridges and walls for defense. That was how the famous Hadrian's Wall was built too. Among the Celts themselves inequality began to grow - the tribal chiefs and nobility became richer than other members of the tribe. Many of them became officials acting for Rome. Tribal chiefs who submitted were appointed to rule their people as before, but now they acted in the name of the Roman Emperor. The noble Celts adopted the mode of life of their conquerors. They lived in rich houses and they dressed as Romans. They were proud to wear toga which was the sign of being Roman citizens. They spoke Latin, the language of the Romans. But the rank-and-file Celts went on living in their tiny huts, they spoke their native Celtic tongue and they did not understand the language of their rulers.