Offa was considered sufficiently powerful by his contemporary Charles the Great to warrant bringing into his sphere of influence by the bestowal of gifts. The two kings corresponded on trade between their kingdoms, and marriage between their children appears to have been considered. Offa introduced the ‘penny’ coinage circulating in Francia and copied Frankish usage in including his portrait in the style of a Roman emperor. He may also have been influenced by Frankish example to attend to the moral and spiritual welfare of his people and drew praise from Charles's Northumbrian adviser Alcuin for encouraging ‘good, moderate and chaste customs’. But, as is often the case with early medieval rulers, his interest in the church had political connotations as well. His presidency of a synod of the southern church in 786 attended by two papal legates helped stress that he was the dominant king of southern England. His wooing of the pope resulted in a grant of archiepiscopal status for the Mercian see of Lichfield in 787 and the consecration there of Offa's son Ecgfrith as king of the Mercians later the same year. Archbishop Jaenbert of Canterbury, who was part of the Kentish opposition to Offa, seems to have refused to carry out the ceremony. It appears to have been opposed in Mercia as well. Alcuin was not surprised that Ecgfrith only survived his father by 141 days for it was a judgement on the blood Offa had shed to secure his succession; perhaps a reference to the culling of rival royal claimants.
Ruler of Mercia, a kingdom in England, Offa left behind what is undoubtedly the third most well-known structure of pre-Norman Britain, after Stonehenge and Hadrian's Wall. Like the latter, Offa's Dyke is a line between one nation and its enemies. But whereas Hadrian intended his wall as a form of protection for Roman Britain—a purpose in which it failed miserably, as the fifth-century invasion of Offa's Anglo-Saxon ancestors made clear—Offa built his earthen dyke simply as a line of demarcation.
Offa's birth year is unknown, and his life prior to 757, when he became king of Mercia in southern England, is a mystery. Upon assuming the throne, he proceeded to bring southern England to the greatest degree of political unification and stability it had enjoyed since the Anglo-Saxon period began three centuries before.
When Offa's cousin Aethelbald (r. 716-757) was murdered, sparking a civil war, Offa quelled the rebellion with ruthless use of power. In the process, he seized control of the land and suppressed the smoldering remnants of insurrection both in Mercia and surrounding vassal kingdoms. The result of these efforts was the creation of a single state that ruled most of southern England.
As the first truly significant Anglo-Saxon king, Offa set out to establish diplomatic relations with the two most powerful forces in western Europe at the time: the Carolingian Empire, and the church. Offa and Charlemagne (742-814) had several disagreements, but just before Offa's death in 796 they signed a commercial treaty. Perhaps even more remarkable was his relationship with the pope, who created a temporary archbishopric in Lichfield to offset the power of Canterbury's archbishop. The office of the archbishop was and is the highest office in the English church, though today that church is no longer affiliated with Rome. Because Canterbury was located in realms belonging to Kent, enemies of Mercia, Offa was willing to grant the pope greater authority over the English church in exchange for the creation of the new archbishopric.
Late in his reign, Offa called for the creation of an earthen wall to mark his kingdom's western border with Wales. This wall became Offa's Dyke, which runs for some 150 miles (240 km) from the Dee estuary in the north to the River Wye in the south. The builders used natural barriers wherever possible, but were still forced to construct 81 miles (130 km) of dyke—a length nearly 13 miles (20 km) greater than Hadrian's Wall.
Whereas Hadrian's Wall was made of stone and garrisoned with soldiers, Offa meant for his dyke simply to serve as a clear line between his realm and the "barbarians" to the west. Offa's Dyke certainly made for a formidable barrier: even today, it is as tall as 8.25 feet (2.5 m) in some places, and with the ditch beside it, is as wide as 65.5 feet (20 m). The wall, representing the work of thousands of men, runs perfectly straight for miles at a time, a testament to Anglo-Saxon engineering skills.
Another technological contribution of Offa's reign was his establishment of a new form of coinage. Coins minted by the Mercian kingdom bore the king's name and image, along with the name of the government minister who was responsible for ensuring the quality of the coins. This tradition continues even today on U.S. paper currency, which bears the signature of the treasury secretary, and the rules of coinage established by Offa prevailed in Britain for many centuries following his death.
Offa's Dyke the name given to a series of earthworks marking the traditional boundary between England and Wales, running from near the mouth of the Wye to near the mouth of the Dee, originally constructed by Offa in the second half of the 8th century to mark the boundary established by his wars with the Welsh.