Observation post in the Bulgarian merchant harbour Varna according to an American CIA report dated 7 April 1954

An item reported that in the commercial harbour of Varna, Bulgaria near the railroad station a circular shaped stone made tower with a height of 20-25 metres was used as an observation post. Continuously was she manned with two men. The post was fitted out with a telephone line.

Source
The report was published on www.archive.org, document number CIA-RPD80-00810A000500340010-8.