Born in Brandenburg-Prussia, Tettau was orphaned at an early age.
He joined the Dutch army as a cadet at the mere age of 13. This began his long career as a soldier, during which he gained a wealth of military knowledge, especially in the art of fortifications.
In the 1680s he entered the service of the Danish King and was employed to supervise the building of several fortresses in Norway.
In 1689 he was promoted to the rank of Major-General and was placed in command of the Danish Foot Regiments sent by the King of Denmark to fight for the newly crowned William III. These forces were quickly deployed to Ireland.
During the campaign to secure the Glorious Revolution, Tettau’s knowledge of fortifications became invaluable, especially during the sieges of Cork, Kinsale, Athlone and Limerick.
At the conclusion of the war in Ireland he was deployed to the European theatre and was wounded at the Battle of Steenkerque in 1692. This was a major defeat for William III and the Grand Alliance. Nevertheless, he recovered from his wounds and continued in service until the end of the Nine Years’ War, after which he retired to his estates in Prussia.