Geograph - photograph every grid square

The ruins of St. Leonards Hospital only hint at the significance of the Hospital in medieval York. Founded soon after the Norman Conquest, it was believed to be the largest medieval hospital in the north of England.
Remains of the hospital's undercroft can be accessed from the Museum Gardens, just to the right of the Museum Street entrance and contains some Roman and Medieval stonework.
The hospital was erected on the site of the former hospital St. Peters which was severely damaged in a fire in 1137. It was closely associated with the Minster, sharing the same grounds because it was so large. It was a self-sufficient building until the Reformation resulted in the religious aspects of hospitals being victimised and consequently St. Leonards was largely destroyed. This left York without a hospital from the time of Henry VIII to 1740.
Grade I listed. Link

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link