The A1210 is a short A-road in central London. It forms the boundary of the City of London for almost its entire length and is wholly part of the Aldgate One-Way System; there is therefore no reason for the road to have its own number but it is specifically mentioned in the list of roads maintained by Transport for London.
The southern end of the road is at traffic lights at the northern end of the A100 Tower Bridge Approach. It heads north along the dual-carriageway Mansell Street, which is the only two-way section of the whole A1210. In fact the road has one lane in this direction and three southbound, as that is the route from Aldgate to Tower Bridge whilst this direction is only for local traffic: at the next set of lights northbound traffic must either turn left onto Shorter Street (and end up in the same place it would if it remained on the A100) or turn right onto the unclassified Royal Mint Street.
The next block is southbound only and is largely in tunnel as it passes under the approach to Fenchurch Street and Tower Gateway stations. After meeting Goodmans Yard, which carries both northbound and southbound traffic on the one-way system the A1210 continues northbound along Mansell Street. Braham Street to the right used to be part of the one-way system but it is now pedestrianised. The A1210 ends shortly afterwards at traffic lights. The A11 turns right, or traffic can turn left to remain on the one-way loop, now numbered A1211.
The A1210 was unclassified in 1922 as its route lies within the City of London, which had no classified roads at all to begin with. It is presumed the number came into being early on as it is the next A12xx number available after the 1922 allocation.
When originally classified the A1210 continued ahead from here along Middlesex Street to end on the A10 Bishopsgate; this is now unclassified and the nearby A1211 performs this duty.