 Like the stations of the South Shore Line (the Braintree branch of the Red Line), built in the same period, the original Haymarket North Extension stations were constructed in a Brutalist style, with angular concrete forms.Community College and Sullivan Square, enclosed under the I-93 viaduct, are primarily rectilinear forms; the later stations incorporate angled forms for staircases and elevators as well.In the early 2000s, Somerville began planning the Assembly Square development, built on the abandoned site of the former Somerville Assembly automobile plant.