For the most part, medieval London relied for its protection on the Roman-built walls that formed a rectangle around Londinium. Most of the medieval gateways were on the site of Roman gates: Ludgate, Newgate, Aldersgate, Cripplegate, Bishopsgate and Aldgate (to take them clockwise from the west side of the city). The walls did not mark the boundary of the medieval city, several of whose wards extended well beyond the walled line, while one – the originally the soke of the Cnihtengild – lay entirely outside the walls. However, the walls symbolically defined the city; they were featured on a communal seal and in a rare medieval illustration of an English town; the areas of the wards which lay outside them came to be viewed as suburbs.

The south-western section of wall was compromised by the construction of Baynard's Castle and Montfichet Castle and their demolition and replacement in the 1270s by a Dominican friary; with financial support from the friars, the city rebuilt this stretch of wall further west, running between the friary and the Fleet River; this was the only major wall-building effort undertaken during the Middle Ages, requiring many decades of stop-and-start efforts. The south-eastern wall was likewise breached by the Tower, which initially incorporated a stretch of city wall, then tore it down as the Tower was expanded. A postern gate was built at the point where the surviving city wall met the Tower grounds. The southern wall, which probably ran close to the line of the later Thames Street, is mentioned by FitzStephen as having been undermined by the tidal effect of the river before his time. Archaeology has confirmed this and also found indications of deliberate demolition around the late twelfth century; this may have been due to scavenging (in 1310 the king condemned those who removed building materials from walls or gates) or perhaps to the extension of port facilities and associated mercantile buildings along the Thames. The latter, which mainly took place in the thirteenth century, may have stifled any desire to rebuild a wall here; instead efforts were later made to fortify the wharfs. The name Billingsgate remembers an opening made in the southern wall, at some early date, to give access to the waterfront.

During the Middle Ages, most of the work on the walls was in the way of repairs and maintenance. A wide ditch was dug, or re-dug, around the walls during the unsettled years of King John's reign (but thereafter became notorious as a dumping-ground for waste), and during the same period Ludgate was rebuilt and the walls were strengthened by the construction of some additional bastions. London Bridge also had a gate installed around its mid-point. London's Barbican complex recalls a medieval effort (prior to1300) to construct additional defences outside Aldersgate; by 1377 it was expected that each gate have a portcullis and barbican, although we do not know if this was or became a reality. Another small gate (Moorgate) was added in the early fifteenth century, to give access to land reclaimed from the marsh beyond the northern wall and to the hamlets further north.

Other than when they deal with provisions for deployment of guards at times of threat or perceived threat, London's letter-books contain only scattered references to the walls and gates, giving no sign that their maintenance was a matter of continual concern or cost. More information on work on the walls would have been forthcoming from financial accounts, now largely lost. Extracts from a murage account (ca.13332) copied into a letter-book show revenues of £235.13s.4d over a two year period, On this only £40 was expended for repairs to the city walls and one of its prisons; and the rest was spent on construction of the Guildhall chapel, gifts to influence persons of power, and other miscellanea. Apart from reconstruction of the Blackfriars stretch (for whose subsequent maintenance the Dominicans were responsible), serious attention to the state of the walls may have been limited to when there was fear of attack. In the 1370s there was some repair activity and it was ordered that every householder work, or send a substitute labourer, to fix up city ditches, conduits and walls for one day every five weeks. Communal labour does not seem to have been called on much for wall work, although it was occasionally for cleaning out the ditches. The city authorities relied on a mix of murage, taxation, and donations to finance wall maintenance. In the 1470s more substantial repairs were undertaken to the northern stretch of wall, using brick, and two of the gates along that stretch (Bishopsgate and Moorgate) were rebuilt; this programme was probably in response to a general sense of threat during the Wars of the Roses, and particularly assaults on the city in 1460 and 1471.

We know of plans to rebuild the wall beside Cripplegate ca.1334, but it is less clear whether they were carried out; part of the aim at that time was to repair Cripplegate and Aldersgate, roof them with lead, and install beneath the gates facilities for lodging the gatekeepers. In August 1375 gatekeepers for all of the gates, including the postern, were sworn in, but it is not clear that such officers were a regular feature of the city bureaucracy. At some other times city sergeants were doubling in the role; for instance, in 1383 custody of the Bridge gate was committed to one of the mayor's sergeants, for as long as he should hold the latter office..

The roofing of the gates, mentioned above, may have been connected with a growing trend to seek income from the city defences, by leasing rooms in the upper levels of the gates, or properties adjacent to gates of walls. Such leases seem typically to have included obligations for maintenance and cleaning by the tenants, extending to roadways underneath or beside the gates. The ten-year renewal, in 1375, of a jeweller's lease on a stall beneath Ludgate, from which to sell his wares, at 40s. a year, appears an isolated instance, although the chamberlain's account for 1339 refers to city-owned shops built beside Aldersgate and Cripplegate. More common was for rooms inside the gates to be granted to civic officials (below the executive level) as perks of office or pension components. For example, in September 1375 one of the city sergeants, who had earlier that year received a thirty-year lease, at 10s. a year, on a garden beside the wall in Tower Ward, was given a life lease of a dwelling on the upper level of Cripplegate, together with a stable attached to the gate, on condition he use part of the space as a lock-up for prisoners sent thither by the authorities; by contrast, when the poet Geoffrey Chaucer took a life lease on a multi-room living space above Aldgate in 1374, it was on the guarantee that the Corporation would not use any part of it to house prisoners during his tenancy . On the same date, a second sergeant was given a life lease of buildings beside the Tower postern (again apparently rent-free), together with the custody of the gate. The following month another bureaucrat was granted a residence above Aldersgate, with a garden associated, for as long as he was in the office of Countor; he was promised that the chamberlain would have all necessary repairs made to the property, at city expense.

There were mixed feelings within the community about non-defensive uses of the city fortifications. The concerns appear to have been that the defences would fall into disrepair or be inaccessible at times of need; the collapse of the postern in 1440 was probably due to neglect. In 1442 a by-law required tenants of city gates to keep them in repair, although this probably only generalized a clause customary in leases. In October 1386 a resolution by the Common Council asked that no grants henceforth be made of city gates, residences over the gates, or plots of land adjacent to the city walls, gates or ditches; after the termination of current leases, custody of those properties should remain in city hands. Yet the very next day Mayor Brembre granted a private citizen a life lease on a dwelling-place over Aldgate, and we continue to find further such leases or grants in the decades that followed. However, we also start to find contractual clauses that tenants should return custody to the authorities at times when the city had to be defended. It was stated in 1435 to be a custom or by-law that no buildings constructed near the city walls or gates should be closer than 16 feet from them, with the intervening land being considered common soil; the reason is indicated by orders given in 1347 for a private structure built too close to the to be pulled down, on the grounds that it interfered with perambulations (probably referring to inspections of the defences, although convenient access for defenders must also have been a concern).

The backbone of the defensive system may have been the city walls and gates, but its lifeblood was the city watch. Like militia musters, the watch arrangements might be necessitated by particular threats, but was also a matter of city routine. Watches were mounted as a matter of course at festive seasons when celebrations could become boisterous (sometimes with intentional elements of "misrule"), and the raised level of public presence in the streets gave opportunities to criminals and created fears about risk to public order.

The first document above is extracted from a compilation of administrative documents, made ca.1215. Round places the text on f.106 of the manuscript, but Bateson ["A London Municipal Collection of the Reign of John," English Historical Review, vol.17 (1902). 505] more credibly places it at f.166. It provides an early reference to the organization of the civic watches, at a time when the sheriffs of London appear to have played a greater role, the ward aldermen having taken the lead later. It was more practical to have the watches organized on a ward basis, and historians have often argued for a close connection between the development of military and security functions and of the wards as territorial units of administration. However, the document may be misleading; quite possibly originating at a time before the mayoralty was established (or at least had received royal approval), it was natural enough for overall responsibility for city defence to be on the shoulders of the earlier city executive, the sheriff, over the election of which official the citizens had only relatively recently gained control. Provisions put in place ca.1285 throw more light on how the watch was put together.

The second document, which was overlooked by the later finding-aid compiled for the Liber Albus, provides a few intimate details not given in later sets of provisions for city defence.

Apart from their personal armour and armament, the city's defenders had, by the Late Middle Ages, some heavier defensive weaponry, city-owned, at their disposal. A bretask for storing some of them was constructed over the Thames, near the Tower, in 1339. An entry in Letter-Book F, undated but probably from about the same time, indicates that 7 springalds, with spare parts (8 bows and 29 bow-strings) and missiles (880 quarrels, some with metal points) were kept there. In some building just outside Aldgate another springald was kept, with a small supply of parts and missiles. In one of the rooms in the Guildhall were held 6 hand-guns, with 5 stocks (to support the guns when firing) and a supply of small balls and gunpowder; also (according to the chamberlain's account of that year) more springalds. In the provisions for defending the city ca. 1377 there is passing reference to ordnance positioned on London Bridge, but we it is not clear whether these were guns or springalds.