Latin mercenaries armed with crossbows and swords.
In the latter life of the Empire, Latin merchants often made their homes within the Byzantine cities they did business in, often taking Greek women as their brides. Their children were known as Gasmuli. Gasmuli had the reputation of being very skilled sailors and fearsome marines. They often took part in the city's defence as militia, often using crossbows and the swords as their weaponry.
The Gasmouloi were the product of mixed marriages between Byzantines and Latins. Pachymeres and Gregoras call them people of mixed race and excellent soldiers, whose military skills combined the prudence of the Byzantines with the boldness of the Latins. They manned Michael VIII’s fleet and they had considerable successes in the Aegean Sea against the naval forces of the Latins who were established in the Aegean islands after the Fourth Crusade. When Andronikos II, following the advice of his counselors reduced the size of the fleet, their role in the Byzantine fleet seems to have declined. However, they did not disappear. Probably, they played a significant role in the civil war of 1341–1347. The sources do not specify whether the Gasmouloi were paid only during military operations or not. That they were permanent residents of the empire means that we cannot exclude the possibility that they received their payment only for the purposes of the campaigns they participated. The Cretan refugees were established in Asia Minor by Andronikos II sometime before 1295. They were to receive annual grants and they provided the army with cavalry troops. They disappear from the sources after the suppression of the rebellion of Alexios Philanthropenos in Asia Minor; although they instigated it, they later opposed and captured Philanthropenos. The Alans were recruited in 1301 and the following year they were sent to Asia Minor. A part of them was sent to Bithynia under the command of Leo Mouzalon and another part was sent to campaign in Magnesia under the orders of Michael IX. The above-mentioned groups of people provided the army with mercenaries, whenever they requested to do so. It seems logical to conclude that, since they were permanent residents of the empire, they also possessed lands, which provided them with their income during periods of peace. Therefore, while these groups consisted a permanent source of mercenary troops for the state, their soldiers were recruited on a casual basis.
Following the Fourth Crusade, mixed unions between Greeks and Latins occurred to a very limited extent when the Latin Empire and the other Western principalities were established on Byzantine soil. The term gasmoulos itself is of unknown etymology and first appeared in the second half of the 13th century. It is, however, not unlikely that it has some relation with the Latin word mulus, "mule". Although it was generally used to refer to children of these mixed unions, it more specifically designated the children of a Byzantine woman and a Latin (often Venetian) father. The Gasmouloi were socially ostracized and distrusted by both the Byzantines and the Latins, who distrusted their ambiguous identity. In the words of a French treatise of ca. 1330, "They present themselves as Greeks to Greeks and Latins to Latins, being all things to everyone...". In a treaty signed in 1277 between Michael VIII and the Venetians, the Gasmouloi of Venetian heritage were considered as Venetian citizens, but in subsequent decades, many reverted to a Byzantine allegiance. As some of their descendants in turn wished to reclaim their Venetian citizenship, the issue of the Gasmouloi would plague Byzantine-Venetian relations until the 1320s.
After the recovery of Constantinople by the forces of Michael VIII in 1261, the Gasmouloi were hired by the Emperor as mercenaries. Together with men from Laconia, they served as lightly armed marine infantry in Michael's effort to re-establish a strong "national" Byzantine navy. The Gasmoulikon corps played a prominent role in the Byzantine campaigns to recover the islands of the Aegean Sea in the 1260s and 1270s, but after Michael VIII's death, his successor, Andronikos II Palaiologos, largely disbanded the navy in 1285. Denied of any remuneration by the Emperor and out of work, some Gasmouloi remained in imperial service, but many others sought employment in the Latin and Turkish fleets, as hired bodyguards for magnates, or turned to piracy.
By the early 14th century, the notion of gasmoulikē douleia ("service as a gasmoulos") had lost its specific ethnic connotations, and gradually came to refer to any service as a lightly armed soldier, both on sea and on land. In this capacity, Gasmouloi served the Byzantines and Ottomans in the 14th century, and the Latin principalities of the Aegean (where the servitio et tenimento vasmulia was hereditary) in the 15th and 16th centuries. The Byzantine navy, such as it was during the empire's last century, continued to use their services. The Gasmouloi played a role in the Byzantine civil war of 1341–1347, fiercely supporting their commander, the megas doux Alexios Apokaukos, against John VI Kantakouzenos. After the latter's victory, many of the Gasmouloi of Constantinople must have been dismissed. Those of Kallipoli eventually joined the Ottoman Turks, providing the crews for the first Ottoman fleets