When was the Brandenburg Gate of Berlin first constructed and what it was used for?

It was commissioned by King Frederick William II of Prussia as a sign of peace and built by Carl Gotthard Langhans from 1788 to 1791. The Brandenburg Gate was not part of the old fortifications but one of 18 gates within the fiscal excise wall , erected in the 1730s, including the old fortified city and many of its then suburbs. Between 1788 and 1791 the prior simple guard houses siding the gate were replaced by the current construction.