The Pennsylvania Power and Light Company Building At Night. It is the tallest building in Allentown. At the time of its completion in 1928, Allentown's population was reaching 100,000, and predictions were made that the city would double in size within 25 years. The Jubilee Parade of 1929 was held to celebrate reaching the 100,000 mark in population, however the population leveled out a just over 100,000 people in the decades since.