The Romans changed Britain in so many ways, from Celtic farming settlements dotted around the countryside and no large settlements other than those of trade, the Romans introduced coloniae, municipia and native civitas must be clearly classified as towns, but many small settlements still existed. Clearly these small settlements grew up outside the new Roman forts. These unplanned settlements arose in a variety of ways, as a result of varied economic and social needs. Some grew out of native settlements, others from settlements outside the new military posts. When the soldiers moved on, the new settlement would still continue. There was also permanent stations such as the house-steads on Hadrian's Wall. Others developed around posting-stations on the Main Roman Roads, where changes of horses and accommodation were made available, others again might be stimulated by market-centers and similar economic needs, while some might have a more specialized origin, such like Bath, whose hot waters enabled what was at first perhaps a small Celtic center to grow into a prosperous Roman Spa.