The coexistence of developed areas and historic buildings is an important topic in urban planning. Our study focuses on this topic from the perspective of urban microclimate. A multi-physics CFD simulation is applied to model urban microclimate with anthropogenic heat and buoyancy effects. First, we clarified the impact of new development, i.e., high-rises on pedestrian-level air flow by comparing city structures in Case A (the past, 1960s) and Case B (the current, 2020s). The results showed an average wind speed decrease of 43% over time. Second, we assessed air temperature increments from anthropogenic heat emitted from Case C (high-rises), Case D (historic buildings), and Case E (both). We found that the mean air temperature increased by 0.16 °C for Case C, 0.52 °C for Case D, and 0.87 °C for Case E, respectively. Third, we developed heat mitigation strategies based on the assessment results in the previous steps. The integration of open spaces and building porosity, which create wind corridors together, can promote outdoor ventilation and heat dispersion in the study area. Compared with Case E, the three mitigation cases improve outdoor thermal environment, with mean temperature reductions of 33%, 25%, and 21%, respectively. Finally, we developed new mitigation strategies by considering the constraints in this special region, where modernity and history coexist. Our practical mitigation strategies will aid urban planning and support conservation efforts not only in Singapore, but also in other tropical and subtropical cities.