Genesis 34:20

‘And Hamor and Shechem his son came to the gate of their city, and consulted with the men of their city, saying.'

The gate of the city was where the leading men would meet in dealing the city's affairs. Hamor could not just dictate terms. Most petty kings were subject to the guidance of their counsellors and had to take their people along with them in major decisions. Thus although he and his son have agreed to the terms they now have to carry their counsellors along with them.