Cruls was born in 1848 in Diest, Belgium, the son of Philippe Augustin Guillaume Cruls (a civil engineer) and Anne Elizabeth Jordens. From 1863 to 1868, Cruls studied civil engineering at the University of Ghent. In 1869 he undertook training as a military engineer and officer, graduating as a 2nd Lieutenant. Cruls served in the Belgian army, attaining the rank of 1st Lieutenant, until 1873 or 1874 (sources disagree).