Complex sentence: To prevent smuggling and enforce the tax it became necessary to erect wooden walls around the city and city gates and toll booths on the major thoroughfares, Mejlgade and Studsgade.
Split into simpler sentences: To prevent smuggling and enforce the tax it became necessary to erect wooden walls around the city. City gates and toll booths were constructed on the major thoroughfares, Mejlgade and Studsgade.