Hello how may I help you? What is tallest building in Denver? It's a republic Plaza would you like to hear more? Where is that exactly? It's in downtown Denver. In Colorado ? Yes. It is located in downtown Denver and stand 714 ft would you like to hear more? Yes sure It is the largest mountain in the Colorado Rockies area it is 5 ft taller than a second tallest building in Denver which is 709 ft The largest tower in the Colorado Rockies area More info. It was built in 1984 contains 56 floors.