In the later Middle Ages, Scottish armies were still largely assembled on the basis of common service and feudal obligations, with the addition of troops maintained by money contracts of "bonds" or "bands of manrent". Common service theoretically called all men between the ages of 16 and 60 for a maximum of 40 days in one year. Such troops were expected to serve at their own expense and to bring their own supplies, a factor that severely limited the ability of Scottish armies to take part in sustained campaigning. Feudalism had been introduced to Scotland in the twelfth century, meaning that knights held castles and estates in exchange for service, providing troops on a 40-day basis, particularly heavily armed noble cavalry. Bonds of manrent were similar to English indentures of the same period, used to retain more professional troops, particular men-at-arms and archers. Scotland relied on these systems longer than was the case in England. In practice, forms of service tended to blur and overlap, and major Scottish lords continued to bring contingents from their kindred. In 1513 for the Flodden campaign these systems were successful in producing a large and formidable force, but in the religious and politically divided mid-sixteenth century there is evidence that the authorities were experiencing increasing difficulty in recruitment.