This caught my imagination as a child like few other places. I could almost sketch it from memory. Originally a 17th (some say 19th) granite tower which was supplemented by the concrete & brick additions during the World Wars. It's real name is the "Grain Tower", but No1 the Thames seems to fit nicely. I've had some memorable times there. Accessible for a very short while at low tide, and don't wear your best sandals.