Long considered an ugly Nazi relic, a half-destroyed concrete fortress in Berlin now has become an addition to the German capital's tourist map.

Since April regular guided tours have taken curious visitors into the vast World War II structure to see the turret interiors and the effect of two failed attempts to blow it up after the war.

It is a part of a growing trend in Germany to show a broader view of the war and include German suffering after years of sole attention to the evils of the Nazis.

Tours pass thick walls that resisted bombs and Russian artillery, bare halls and staircases where civilians sheltered and deep shafts which carried anti-aircraft shells from the basement to the rooftop guns seven floors above.

Visitors can also marvel at technology well advanced for its time. The gun steering, for example, was fully automated. A radar tower 300 yards away tracked enemy aircraft and fed signals along cables still visible clinging to the walls.

The fortress is one of six that Adolf Hitler ordered to be built in the German capital to defend it from air attack. His command in September 1940 came just days after Berlin came under a three-hour barrage from Allied planes.

Hitler himself sketched the form the defenses should take with 120-foot-high turrets and guns at each corner.

Financial constraints eventually limited the number to three fortresses, completed by April 1942, although two further structures were built in Hamburg and Vienna.

Each complex could hold around 15,000 civilians and their 8 foot-6-inch walls were deemed impenetrable.

The post-war Allied occupiers in Berlin decided to destroy most military structures. The British and Russians managed to bring down two of the complexes after several failed attempts.

However, the French were unable to destroy the fortress in their northern Berlin sector, leaving two towers and 1.6 million cubic yards of debris. The latter was partly landscaped, but the remaining structure has been largely untouched for 50 years.

The Berlin Underworlds Association already runs tours of nearby wartime and Cold War shelters, but preparing the half-demolished air defense fortress for visitors was a task of a different order. It took thousands of hours of volunteer labor to ready the building for show.

The bunkers may not be so well visited as the glass dome on the Reichstag, Germany's parliament building, but interest is growing. Last year 25,000 visited the site compared to 8,000 in 2001.

(agencies)