The most well known buildings in London which are medieval (omitting the tower of London and Westminster abbey) are perhaps the city churches. Before the great fire of London the city had 109 churches. This was reduced in the great fire and during later centuries due to rebuilding works and heavy restoration.
Church building in the city began in earnest after the Norman conquest with the establishment in the twelfth century of Augustine priories (such as St Bartholomew-the-great and Southwark Cathedral) and knight houses (Knights hospitaller of St John [church of St John] and knights Templar [Temple church]) who built their churches just outside the city walls. In the fourteenth and fifteenth centuries the expansion of these monasteries was halted and in the aftermath of the dissolution of the monasteries the monastic churches were put to new uses as large parish churches or left to ruination. Very few new churches were built in the Tudor and early Stuart periods of the sixteenth and early seventeenth centuries (although Katherine Cree is a notable exception).
The great fire of London spelt the end for many of the medieval churches in the city, with 89 churches destroyed. Of the remaining medieval churches many were demolished and rebuilt during the following centuries. During the nineteenth century many of the surviving churches were restored or sometimes radically rebuilt by over zealous Victorian restores (the evidence is visible in most city churches). The following twentieth century saw continuing destruction firstly by the blitz in which many churches were gutted and secondly by a IRA bomb in 1993 which badly damaged two of the best preserved medieval churches in the city (St Helen's and St Ethelburga).
St Giles Cripplegate +
St Andrews Undershaft
St Ethelburga +
St Helen's Bishopsgate *
St Olave +
Outside the city walls
Southwark Cathedral *
Temple church + *
St John of Jerusalem (crypt) + *
Other city churches (with many later additions)
Bartholomew-the-lesser (tower) +
All Hallows by tower +
St Sepulchre-without-Newgate +
St Katherine Cree (tower)
(remains of) All Hallows staining (tower and crypt)
* - City churches most worth seeing
+ - Much restored churches
other city churches with minor medieval features
St Mary Le bow (Crypt)
St Andrew Holborn (part of tower)
St Bride fleet street (Crypt)