His military capital was at Sutton Walls near Sutton-St-Nicholas, and it is probably that Hereford was his civil one, hence Offa street.
The name Mercia means boundery and indeed Offa's kingdom straddled the Welsh border with the city of Hereford on the very frontier. Offa built the first defences for Hereford after it was attacked by the Welsh in 769. These consisted of earth and timber banks surrounded by a deep ditch, the outline of which is marked in the subway at Eign gate.
These structures can still be traced today, but Offa's greatest legacy was the monumental defensive earthwork he constructed between England and Wales, famous as Offa's Dyke to try and keep out raiding parties and possibly also as a springboard from which the Mercians could launch attacks Wales.
Apart from where rivers supplied a boundery, this dyke ran 120 miles from Clwyd
to the Severn estuary. It was a 6-feet-deep trench fronting a rampart 19ft high.
Garrisons were positioned at regular intervals along its length.
With some 80 miles of it still surviving, it is the largest earthwork of its
type in Europe.
Under Offa Mercia became the overlord of the Anglo-Saxon kingdoms south of the Humber, and he styled himself as Rex Anglorum (King of the English). He gained control of Wessex through alliances of marriage and Kent and Essex by military might.
London grew as Offa strengthened trading links with the continent, including with the Holy Roman Emperor, Charlemagne. Liason was establish between the two kings with mutual esteem until they fell out over marriage proposals concerning their children.
Offa coined the first silver pennies which were such a high standard of weight, size and artistry that they set the basic for English coinage for the next 600 years.
Interestingly he minted some coins with the inscription 'There is no God but Allah'. Some have speculated whether he was a convert to Islam though this is very unlikely, these were probably made for export, as in other ways he acted like a Christian king. He held a synod in Chelsea ca. 787,and granted land at Harrow to the Abbey of St Albans which he founded.
Unfortunately one dark event casts a shadow over his reign. In 872 King Ethelbert of East Anglia travelled to Sutton Walls hoping to marry Offa's daughter, Elfrida. Ethelbert was the son of Ethelred and Leoverorina, and was described as being a 'well-favoured youth' and a most devout Christian. On becoming king, his nobles persuaded him to marry for the sake of the kingdom, so he decided to see out Elfrida's hand in marriage. The night before he arrived, Ethelbert encamped on Backbury Hill, near Fownhope, the lines of the eponymous St Ethelbert's Camp still remaining faintly traced even after the lapse of a thousand years. However, Offa's queen, Quendreda felt hurtful towards Ethelbert. Perhaps because she felt that such a marital alliance might make Ethelbert powerful enough to replace Offa as the greatest ruler in England. Another account is that Quendreda had made advances towards Ethelbert, but when he spurned these, shey was piqued. She persuaded Offa to have Ethelbert murdered. Offa offered 'large money' to any of his men who would do this dirty deed. Quendreda then proceeded to drug Ethelbert's wine, and then a man by the name of Gunbertus (or possibly Gimbert, Ginnbert or Winebert) beheaded poor Ethelbert with a sword. Offa had him buried beside the Lugg. When Elfrida heard the terrible news, she vowed herself to virginity, and entered a convent after foretelling of both Quendreda's death and that of Quendreda's young son.
However, that night miracles occurred where the dead king lay at Marden. In the natural order of things in those days a well of clear water sprang up there, over which a church was erected, as may be seen this day by the banks of the Lugg, although nothing survives of the early church, but it the floor is the well of St Ethelbert which has been dry for several years. Offa felt guilty and had Ethelbert's remains moved to Hereford Cathedral and built a shrine to house them. Incidentally, at that time, the bishops of Hereford had connections with Bosbury, where they 'held their state.., and dwelt in a fayre palace in the time of King Offa'. The spring which afterwards supplied the castle s said by pious tradition to have arisen. This can still be seen today as St Ethelbert's Well in Redcliff Gardens. Ethelbert was made a saint, his tomb attracted pilgrims whose money couple with donations from Offa allowed the Cathedral to be rebuilt far more grandly. As a further sacrifice of penance he gave the patronage of Wood street, London and land at Stanmore to the Abbey of St Alban's.
Also the prophesy that Elfrida had made about the death of Quendreda came true two months after Ethelbert's murder.
Hinton or 'Hinniton' in the Golden Valley was the home for monks brought to Peterchurch by Offa to found the church in the area. In fact Hinniton means 'monks' settlement', and is now Hinton Court. As Offa pushed the boundary of his kingdom further towards the west, he introduced Christianity. Offa was powerful enough to apply to the Pope for an archbishop for his own lands, so the Bishop of Lichfield was elevated to that rank. The group of missionaries that set up their headquarters at Peterchurch were high standing, and even had an Italian bishop from Rome, Theophylact, amongst their number, a Papal Legate who was Offa's spiritual adviser. This connection with Rome is why the church built of stone from the start even at the time when Hereford Cathedral was made of wattle, daub and timbers was named after St Peter. Even today's church there is similar to the type of church that was fashionable at Rome during the era when it was first constructed.