Magento – City field not required on checkout in Magento 2

If you plan to make the second field mandatory for the checkout, you should make it mandatory in the customer add/edit address form for consistency.

For the client side validation you need to edit each template that holds an address for and add required-entry class on the second row. Here are the templates I found for this:

• customer/form/address.phtml
• customer/address/edit.phtml
• persistent/customer/form/register.phtml
• customer/form/register.phtml
• checkout/onepage/billing.phtml
• checkout/onepage/shipping.phtml
• paypal/express/review/address.phtml
• persistent/checkout/onepage/billing.phtml

As for server side validation you need to copy the file

app/code/core/Mage/Customer/Model/Address.php to the local folder /app/code/local/Mage/Customer/Model/Address.php. You cannot extend this class because the class is never instantiated.
And you need to add something in the validate method.

After this section:

    if (!Zend_Validate::is($this->getStreet(1), 'NotEmpty')) {
        $errors[] = Mage::helper('customer')->__('Please enter the street.');
    }

Add the validation for the second row

    if (!Zend_Validate::is($this->getStreet(2), 'NotEmpty')) {
        $errors[] = Mage::helper('customer')->__('Please enter the correct street.');
    }

Yes.

The easiest would be to add a new entity where the values for your cities are stored. In the frontend, just change the input type to dropdown and load the list of cities. When a user selects a field from the dropdown, store the text value (as it is now). You could even do a check if the selected city is a valid one from your city list before storing the data. This way you don't have to change a lot.

A more advanced approach (if needed) would be to also add relations to the directory_country and/or directory_country_region tables to only show cities which belong to the specific country or region selected in the checkout.