This is an exceptionally well-built film with a subject more than worth the effort, well delimited and well illuminated. Teaches a lot, by showing, not saying. Truly beyond praise.
Der Tunnel illustrates how it felt to live in East Germany in the years before the Wall was built, how it felt once it was built, and the terrible determination of some people who escaped in the first few days, to make their escape serve others in East Berlin by trying to bring them to the West, even if the attempt should cost them their own lives. The film's primary power lies there, in the testimony value of each of its frames.
I lived for a few months in Germany less than four years after the events shown, within a walk from the East German border. I visited East Berlin several times and traipsed (illegally) into East Germany once. Der Tunnel shows truths I would never have found the words to explain, and shows them in a way anyone can feel, if perhaps by watching the film more than once. Another film that is instructive on some of the same topics is Volker Schlondorf's semi-fictional Legends of Rita, well worth watching after Der Tunnel. (Legends of Rita takes place in the last years before the Wall fell, more than 20 years later.)
One note. In Der Tunnel, the more people are compromised by the East German regime, the more they invoke their conscience and their deep-felt personal opinions. (You might call this the Lutheran variant of Stalinism.) This occurs equally in Legends, except that in the latter the character who is totally "political consciousness"-driven is the West German Rote-Armee protagonist, who does nothing but meaningless murders until she escapes to the East and, there, is kept under control. So, in Legend, the various East German security people come off better with their "conscience" than does the truly-awful protagonist. In Der Tunnel, they don't come out as well. As for the diggers, they never talk about conscience or opinion at all. That they will do everything in their power to save their loved ones from the regime is simply obvious to all and never discussed.
A second note. The only non-East-German involved in the digging is an American undercover operative. Despite working, no doubt, under the same constraints as his opposite numbers, he shows as much determination as any of the other diggers, and perhaps even more courage.
Concerning Marcel's ending query -- The Colonel stops at the border sign because doing anything beyond the sign would lead to major diplomatic embarrassments, and naturally to demotion or worse for him. He certainly can't kill all the witnesses (and has no intention of doing so). This is the same mechanics that explain why the American operative, after being caught in the East by the Colonel's services, is only detained for a spate of days, with no torture nor bad treatment except cold and isolation, and then released. (He carefully avoided engaging in anything like espionage.) These rules were known to all at the time (self included).
