The Savoy Hotel - the Art Deco Court Side entrance (pictured) has traffic driving on the right, permissible by an act of parliament. Monet stayed here and painted many studies of Westminster Palace from nearby bridges & the balcony of his suite. WC2, Charing Cross Tube.
Oystermouth Castle, Mumbles, Swansea, Wales - at twilight, if you try to sneak in, the ghosts shoo you away. It is so much smaller than I had imagined but had a chapel and was a delight to walk through.
The entrance to the Tower Subway - the first 'tube' line in the world. It ran under the Thames but was only open for a few months. This is its north side of the river entrance on Tower Hill, rebuilt after closure. According to Mark Brader, the equivalent south side entrance "...fell to redevelopment" in the 1990s.
St Bartholomew The Great - this is the [reverse side of the] Tudor gatehouse to the priory church, which leads onto West Smithfield. The stone arch is all that remains of the former entrance to the Western façade, destroyed in 1539 during the dissolution of the monasteries. The Tudor half-timber frame was added in 1559. If you do visit, Club Gascon/Cellar Gascon (food), are just through the arch & The Jerusalem Tavern & Ye Olde Mitre (drink) a short wander away. EC1, Barbican Tube.