sounds great what you are planning.
Will you also aim to publish your baby on steam?
Your plans sound very solid and I am looking forward the progress of this project. Even with simple graphics it convinces with its unique charme of the great RTS games back in the 90s :)
Do you have any ideas about economy and resources? The idea of different major cities offer great potential of strategic resources and their trading and distribution all over the empire and potential allies for support ;)