The Assembly Row project team recently celebrated another milestone in the massive mixed use development in Somerville. Executives of the development firm, Federal Realty Investment Trust, and the construction firm, Callahan Construction Managers, joined with other project participants in a “topping off” ceremony for Block 6, the latest phase of the development. Representatives of the architectural firm, Stantec, and the Ironworkers Union Local 7 also took part in the event, at which participants signed the final steel beam before it was hoisted and connected to the top floor of one of the two Block 6 buildings. Assembly Row when complete calls for more than 600,000 square feet of retail, 1,800 residential units, and 2.8 million square feet of office space. Alread,y several portions of the project have been completed and are open. Block 6, expected to be completed in early 2018, will include a 20-story girder slab tower with 273 residential units and a six-story wood frame structure with 174 residential units, along with 40,000 square feet of ground floor retail space and a seven-story, 650-car parking garage.John Laidler can be reached at email@example.com.