This was Herman Wedemeyer's first appearance as Duke. However, it would be a few years before he would become a regular character on the series.
Much of the footage of the siege from this episode was re-used in Season 5, Episode 20, "Little Girl Blue" about a kidnapped girl whose kidnappers apparently take refuge in the same bunker as in this episode.
This episode may have been "inspired" by the events of August 1, 1966, when Charles Whitman, a former Marine (having already killed his mother and wife the night before) brought an arsenal of weapons to the observation deck on the 27th floor of the Main Building of the University of Texas in Austin. In less than two hours until he was killed by police, Whitman shot and killed 15 people, including an unborn child, and injured 31 others. It was arguably the first such mass shooting in the U.S., and had the worst death toll of such an incident until the McDonald's shooting in San Ysidro, California in July, 1984.
The sniper has only a few short lines, and they are at the beginning, when he is purchasing the rifle.
The f word is clearly readable in the graffiti on the wall of the overpass where the sniper is hiding out. (See images attached to this episode: