Update 2013: The watchtower has been bought by a private investor, restored and is now open to the public Tuesday - Sunday, from 11:00 - 16:00, except for when it's raining.
Hidden in a backstreet by Potsdamer Platz, the past is lurking among all the new buildings...
Behind the high concrete wall which faced the west was the death strip, an area of sand (heavily laced with weedkiller so that no plants would grow which could provide cover for escape attempts) and various defences intended to make crossing the area next to impossible. In case anyone still thought it would be a good idea to try - and plenty did - border guards were on the lookout from these watchtowers, on orders to not hesitate to use a firearm if necessary, even if women and children were involved. On the other side of the death strip was another wall.
This particular watchtower was situated outside the death strip, intended for watching people approaching the eastern side of the border. It's actually been moved a few metres to make way for the new buildings here on Erna-Berger-Straße, but it's now a listed monument, the last watchtower of its type (BT-6 for all you watchtower fans out there) still standing in Berlin. This type was designed to give a good view through all 360 degrees around it.
Approaching the tower this time, I noticed that someone had tried to force the metal door open. The lock hadn't given way, but the corner was bent. I turned on the flash on my little camera and stuck it inside to see what I could see.
Not the world's most interesting photo. You can see the metal ladder leading up to the top - a pretty narrow space. They replaced these with larger, square watchtowers later, as it was difficult for the troops to get out quickly if they needed to.
There used to be a small section of the wall remaining on the corner where Erna-Berger-Straße meets Stresemannstraße. It's gone now, unfortunately, so this watchtower is now all that remains of the traces of the border in this area.