The Erna-Berger-Straße watchtower seems to find its way into a fair few walking tours and tourist guides and I've caught some people saying it's the only surviving watchtower in Berlin. Naughty. There's only one of its type, but there are three watchtowers in central Berlin...one of them is this one, hiding among the trees of the Schlesischer Busch park just off Puschkinallee:
Maybe hiding is too strong a word - it's fairly easy to spot - but it's fair to say there are quite a few trees around it. The trees to the south-east of the tower were left standing on the East Berlin side of the wall, and simply designated a 'Sperrgebiet' (no go zone). To the north-west, the trees were cleared along the strip parallel to the Flutgraben canal, creating a death strip that the tower looked out into.
This particular type of watchtower was a Führungsstelle - a command post - once one of 31 along the course of the Berlin wall. As well as being used as an observation tower, its occupants were also responsible for the command of border troops in watchtowers and other facilities in the surrounding area. All the signals from the sector's signal fences came through this watchtower first; if somebody triggered the alarm by attempting to get over one, it would sound in this watchtower first, before the troops in the relevant watchtower were notified. Communications were also routed through here, plus it was the only type of watchtower with heating and a toilet!
The tower is closed during the winter, but run as an art gallery by Flutgraben e.V. between May and September.
If you're visiting the watchtower, don't forget to cross over to the other side of Puschkinallee, where there's a small segment of wall (Hinterlandmauer) just in front of the industrial buildings there.