The city as a whole was highly susceptible to fire damage. Many of the industrial buildings also were of wood frame construction. The houses were of wooden construction with tile roofs. Outside the center, the area was congested by a dense collection of small wooden workshops set among Japanese houses a few larger industrial plants lay near the outskirts of the city. The center of the city contained a number of reinforced concrete buildings as well as lighter structures. To quote a Japanese report, "Probably more than a thousand times since the beginning of the war did the Hiroshima citizens see off with cries of 'Banzai' the troops leaving from the harbor." The city was a communications center, a storage point, and an assembly area for troops. It contained the 2nd Army Headquarters, which commanded the defense of all of southern Japan. Hiroshima was a city of considerable military importance. 75% of the population was concentrated in the densely built-up area in the center of the city. There was no marked separation of commercial, industrial, and residential zones. Of a city area of over 26 square miles, only 7 square miles were completely built-up. A single hill in the eastern part of the city proper about 1/2 mile long and 221 feet in height interrupted to some extent the spreading of the blast damage otherwise the city was fully exposed to the bomb. The city is almost entirely flat and only slightly above sea level to the northwest and northeast of the city some hills rise to 700 feet.
The city of Hiroshima is located on the broad, flat delta of the Ota River, which has 7 channel outlets dividing the city into six islands which project into Hiroshima Bay.