Future smart cities promise to dramatically improve the quality of life and have been attracting the attention of many researchers in recent years. The integration of IoT with their corresponding service delivery models to manage a city's asset securely remains a significant challenge. The deployment of diverse IoT technologies and several architectural components and novel entities of emerging ICT solutions opens up new security threats and vulnerabilities. Large-scale, seamless communication among multiple IoT technologies is highly dependent on the operations of the underlying wireless access technologies such as WSNs, SDR, CR and RFID. We present thematic layered taxonomies to highlight the potential security vulnerabilities, attacks, and challenges of key IoT enabling technologies which underpin the development of smart cities. We also identify potential requirements and key enablers that play a vital role in the development of secure smart cities. Finally, we discuss various open issues that need to be addressed to unlock the full potential of 5G for future smart cities.