Defensive architecture in Southeast Asian port cities during the eighteenth century is a topic never addressed from a transnational perspective. This paper aims to analyze it as a phenomenon of scientific transfer, considering fortifications as a remarkable example of “open air science.” First, it shows the complex situation among antagonistic powers in the Malay and South China seas. From here, it aims to identify the connections between Dutch and Spanish proposals in the area. One model focused on protecting sea routes, while the other was more concerned about maintaining territorial integrity. Later, it considers how local kingdoms from China or Siam to the southern sultanates addressed the problem. Here, a variety of answers have been found, ranging from a complete rejection of European solutions to qualified adaptations and wholesale adoption of them. From all these examples, it is possible to evaluate the nature of technical transfer in a transnational perspective.