A large-scale research project to develop a robot-based automated building construction system for steel structures was successfully conducted in South Korea. This paper discusses the results of the real-scale test and the key lessons gained from the testing process. The system was assessed in terms of system productivity, construction cost, quality control, and safety improvements. While the productivity of the automated system showed an improvement of about 9.5%, the construction cost was about six times higher than that of the conventional method. The field test also indicated that the automated system requires more on-site quality control measures. However, because the system can eliminate the causes of various safety accidents, safety levels might be expected to be improved significantly. It is expected that this paper will provide knowledge and insight for developing new systems, and the results of the real-scale test might be useful for other researchers and similar research projects in the future.