Seibu Lions Stadium was constructed in 1978 to mark the inception of the Seibu Lions professional baseball team, and has since been its home ground. In 1999 Kajima designed and built a domed roof for the ballpark, making it a partially open domed stadium that, without an enclosing wall behind the stands, "coexists with the external environment." In 2009, the spectator seating underwent a major refurbishment that included a new design with unique seats and more spacious main aisles and stairways to enhance spectator entry-and-exit flows and integration with adjoining spaces. The latest adaptation without demolition was subject to an array of constraints, including an extremely compressed off-season construction schedule and the limitations of the existing structure. Kajima's specialized technologies underpin the ongoing evolution of the stadium, while its design and construction maximize the features of each renewal.