The North Gate and Bocardo Gaol
Until 1771, the North Gate spanned Cornmarket immediately to the north of St Michael’s Street (until the twentieth century part of New Inn Hall Street) and St Michael-at-the-Northgate Church. Hence Cornmarket was known as North Gate Street until 1536, when a covered area was erected there to facilitate the selling of corn.
This map drawn by Agas in 1578 (Historic England CC51/00442) gives a bird's-eye view of the North Gate, looking south. At this point the gate had battlements and the Bocardo gaol did not extend over it:
This drawing made by N. Calcott in 1770 and engraved by Orlando Jewitt shows the North Gate of the city of Oxford, looking southwards down Cornmarket, just before it was demolished. The tower of St Michael-at-the-Northgate Church (which then had battlements) is on the left, and part of the Bocardo Gaol was now on top of the gate:

A similar view can be seen here (Historic England HT00117):
Anthony Wood described the North Gate and the Bocardo thus in his “Survey of the Antiquities of the City of Oxford” which he composed in 1661–6 (reproduced in Andrew Clark (ed.), Wood's City of Oxford, Vol. 1, pp. 255–7):
This was the strongest gate of the city; as indeed by good reason it should, having no river before it, as the other hath. It was well strengthned on each side with a strong, bulky tower; and backt on with another gate, both formerly well fenced (especially the outermost) with a port-close or port-cullis (as 'tis mentioned) to let downe before; as alsoe a military engine erected over it, through which they could cast downe anything obnoxious to the enemy approaching therunto. Such a gate soe strengthned was called “porta macho-collata,” which was a passage over it like to a grate through which scalding water or any other weighty substance may be cast uppon the assailiants. Beside this there were two great folding dores hung thereon, made strong with barres of iron nailed upon them; as alsoe a massie chaine that crossed the outward gate. By which we cannot otherwise imagine its pristine beauty and strength, not only for fortifications but for battlements statues and armes theron; which afforded great delight to strangers that cam that way.
And soe without doubt it might have continued, but, the Barons’ Warrs ceasing, Halcion days appeared and our swords became rustie; and this place, for want of use, fell into the hands of the Mayor and bailives, who afterwards made it [a] common prison for debters and malefactors belonging to their owne city, which for the same use continueth to this day; and a prison for scollers for little faults….
Now whether or noe “Bochord” or “Bocardo” were put to such uses as a library and place for sale of books when our University was in Bellositum, (i.e., now Bewmont, to which they joyne close to each other, which was before this gate was built or any thoroughfare there; I shall leave it to your sower criticks in antiquityes to chew upon; and proceede onward.
The Oxford martyrs Cranmer, Ridley, and Latimer were all confined in Bocardo in 1555.
An Act of Parliament for improving Oxford’s roads made at Westminster on 10 May 1770 sealed the gate’s fate. It states in its Second Schedule:
To take down the North Gate, and so much of the Prison called Bocardo, and such Houses and other Buildings on both Sides of the said Street, near Saint Michael’s Church, and purchase such Ground as shall be necessary to widen the said Street.
The North Gate was duly pulled down the following year. Jackson's Oxford Journal of 21 September 1771 reported:
Yesterday the Workmen employed in pulling down part of the City Wall, adjoining to Bocardo, for widening the Northern Avenue conformable to the late Paving Act, found, three Athenian Silver Coins of high Preservation; another of the same Coins had been found in taking down the Prison, some Days before.
Bocardo was replaced by a new City Gaol in Gloucester Green, which survived until 1876.
The continuation of the city wall from the North Gate
After crossing Cornmarket, the wall continued on its eastward journey behind the south side of Broad Street, which was then just a ditch outside the city wall. A plaque on the wall of the garden to the left of St Michael's Church marks the site of the wall:

Arthur Pearson, who with his son erected this wall in 1912 on the site of the city wall, was the owner of Boswell’s department store and rebuilt 31 Cornmarket that year.