Since the 16th century, the people of the Netherlands have used their expert knowledge of hydraulic engineering for defense. The Defense Line of Amsterdam is an 84 mile long line built 9 to 12½ miles around Amsterdam by the Dutch government between 1883 and 1920. In the event of an attack on the country, the army, the government, and the king could withdraw within this perimeter. The Line consists of 42 forts and a large number of batteries, dikes, and locks.The Defense Line is an ingenious hydraulic engineering system. UNESCO says it is the only example of a fortification based on the principle of controlling water. The armed forts were designed to act in conjunction with temporary flooding from polders and a complex system of canals and locks. In an attack, Amsterdam would be difficult for an invader to reach because the land would now be under water. The forts still exist, and have had various functions over the years, from storage, or homes, or even prisons.In honor of this impressive line of defense, the Royal Dutch Mint was commissioned by the Ministry of Finance to issue this official commemorative coin. The Fort on the Ossenmarkt is in the coin’s center. The artist chose this fortification because of its complementary round shape in relation to the coin. The forts of the Defense Line are mentioned along the outer edge. The circle with the names represents the 135-kilometer-long line of defense.The .900 fine gold proof €10 limited to 1,000 pieces is 22.5 mm and weighs 6.72 grams, the same as the old 10 guilder. It is in a wood box with certificate.