Huaiyuan Gate marks the western side of Shenyang's old inner city, close to the imperial palace district. The current structure is a reconstruction rather than a surviving Ming or Qing gate, but it gives useful shape to a part of the city where the old walls have mostly vanished under modern streets.
Treat it as a waypoint while exploring Shenhe rather than a destination on its own. It sits near metro access and makes a sensible stop before or after the Shenyang Imperial Palace, especially if you want to understand where the old walled city began and how little of that defensive fabric remains.