For information on on the numbers, please see Owada Tetsuo Okehazama no Tatakai, pp. 86-87 and Hashiba Akira Shinsetsu Okehazama, p. 180. As for the construction quality of forts during the Sengoku Era see Paul Varley "Oda Nobunaga, Guns, and Early Modern Warfare in Japan" pp. 105-125.
The five forts were built around 1559 and they had a purpose.
- Tange, Zenshoji, and Nakajima were to balance out Narumi Castle.
- Marune and Washizu were to counter Odaka Castle.
- Washizu was to counter Odaka and Narumi.
- Marune was to counter Odaka and Kutsukake Castle.
The area of Fort Tange commanded by Mizuno Tatewaki with 340 soldiers.
Fort Zenshoji commanded by Sakuma Nobumori with 450 soldiers.
Fort Nakajima commanded by Kajikawa Takahide with 250 soldiers.
Fort Marune was commanded by Sakuma Morishige with 150 soldiers and Fort Washizu by Oda Nobushige with 520. All numbers are approximate.
Here is a great link with maps, forts/castles, and the campaign route.
Nobunaga no tame!