There is a simpler solution to this. The approach also depends on the some assumption which are listed below:
Assumption : If there is only one City which you want the City field to be set to then you can use jQuery. Using jQuery you will be setting the value of the field and make the fields disabled to avoid change in City field value.
A rough example will be :
jQuery(‘#billing_city’).val(‘Mumbai’).attr(‘disabled’,’disabled’);
You just need to add this line such that it is execute on Checkout page.