The Marienkirche is the oldest consecrated church in Berlin. It was built in the 13th century, first mentioned in 1292 though thought to date from slightly before that. The tower was added in the 15th century, built of Muschelkalk (shell-bearing limestone) from Rüdersdorf (to the east of Berlin). The original spire was quite different; the current copper spire was added in 1789-90 by Carl Gotthard Langhans, who was also the architect of the Brandenburg Gate.
It was then restored and added to in 1893-95 by Hermann Blankenstein - the four-gabled façade on the side is his work. It needed restoration again after World War II, though unlike some of Berlin's churches (the nearby Nikolaikirche, for example), it remained usable even before the restoration began. The area around it was cleared extensively in the late 60s, leaving it standing alone in an area which once was tightly packed with buildings.
I didn't go inside this time, so the interior will have to wait until another post, but it's definitely worth a look. It has a beautiful vaulted ceiling and its walls are home to a late 15th century Totentanz (danse macabre - dance of death). It also has a fine organ, originally built by Joachim Wagner in the early 1720s (though like virtually all organs, it's been rebuilt/restored many times since then, including most recently in 2002).