the main caponier and the half caponiers were constructed in a bunker with cannon cellars. At the rear end the fort could be supplied via a bridge over the canal. To protect the rear two low batteries were placed for ditch defence. The later forts such as the barrier-forts have a similar structure but without réduit. The forts were connected by the R11 military road, thereafter developed into a small green outer ring for Antwerp. Act of 1906 The construction of these forts began in 1909 after the expropriation of land. The works were performed by the firm Bolsée from Antwerp. The forts are very similar, with some exceptions. The forts were built in 2.5 m unreinforced concrete, which would offer resistance to 28 cm caliber shells. Most fortresses are known as so-called second order forts with a main armament turret for two 15 cm guns, two turrets for 12 cm howitzers and four turrets for a 7.5 cm gun. The so-called first order forts had an extra turret for two 15 cm guns and two additional turrets with a 7.5 cm gun. The 15 cm and 12 cm howitzer guns were placed at the main front. On the flanks of the main front caponiers were placed for ditch defence. These are in the following types: composite casemated caponiers, caponiers with turrets (Fort Bornem) and detached (Forts Stabroek, St. Katelijne Waver and 's Gravenwezel) or attached reverscaponiers (Forts Brasschaat and Kessel). Fort Bornem has a different structure with a pseudo-bastioned front with turrets on the caponiers. Armament The forts had a multitude of functions: Long-range defence Fire support between the forts themselves Short range and canal defence The first forts 1-8 (Act of 1859) originally had no fixed artillery. The artillery consisted of mobile field artillery. Developments in the gun caliber had been taken into account in the construction by an increased ground cover. In the later forts fixed artillery is used. The main developments in the field of artillery were: From 1885, instead of gunpowder, picric acid or nitrocellulose was used. The shells had explosive action by using TNT. In Germany, the first drawn-steel gun barrels were manufactured, which replaced cast iron and bronze barrels. Gun caliber, thus, could be increased. The effect of a shell impact increased greatly. Development of chemistry and metallurgy dramatically changed the cannon, which, until the mid 19th century, had barely changed. The artillery