The core of the Great Moravian army was a princely retinue comprising professional warriors, who were responsible for collecting tribute and punishing wrongdoers ("družina"). The "družina" consisted of members of the aristocracy ("older retinue") and members of the princely military groups ("younger retinue"). Some of its members formed a permanent armed guard for the prince, while the rest were garrisoned at forts or at other strategic points. The "družina" was probably relatively loyal and provided stable support for the prince since there is no known record of any dissatisfaction with it or of any uprising. The permanent part of the army had an expressly cavalry character. The Great Moravian heavy cavalry emulated the contemporary Frankish predecessors of knights, with the expensive equipment that only the highest social strata could afford (a contemporary Arab traveller, Ahmad ibn Fadlan, reported that Svatopluk I had plenty of cavalry horses). The overall size of the "družina" is estimated by Ruttkay at 3000-5000 men. In the case of larger mobilisations, cavalry was reinforced by additional smaller units recruited from the retinues of local magnates and from traditional communities ("občina"). The second element of the army ("pohotovosť") consisted of lower classes of free citizens who were not, in most cases, professional warriors. However, thanks to their large numbers and knowledge of the prevalent types of weapons they represented a serious military force. They played a decisive role mainly in the defence of Great Moravian territory; their participation in wars of expansion was less common. The army was led by the prince or, in his absence, by a commander-in-chief called a "voivode". The maximum size of the army is estimated at 20,000-30,000 men. In case of external aggression, ordinary people participated in defence and diversion actions. An important element of the defence of Great Moravia was a system of strong fortifications, which were difficult to besiege with the then prevailing forms of military organization. For example, a Frankish chronicler wrote with awe about the size of Rastislav's fortress (""firmissimum, ut feritur, vallum"").