How to create a custom form field type in a module?

Im useing joomla 2.5, and I want to create a custom form field type that stored in the same module.

In the XML:

 <fieldset  name="basic" addfieldpath="/modules/mod_royalslider/fields"></fields>
      <fieldset name="basic">
           <field name="title" type="City" label="anythging" description=""   />
      </fieldset>
 </fields>

In the file /modules/mod_royalslider/fields/city.php

<?php
// Check to ensure this file is included in Joomla!
defined('_JEXEC') or die('Restricted access');

jimport('joomla.form.formfield');

class JFormFieldCity extends JFormField {

    protected $type = 'City';

    // getLabel() left out

    public function getInput() {
            return '<select id="'.$this->id.'" name="'.$this->name.'">'.
                   '<option value="1" >New York</option>'.
                   '<option value="2" >Chicago</option>'.
                   '<option value="3" >San Francisco</option>'.
                   '</select>';
    }
}

for now it show me error, when ever i remove <fieldset name="basic" addfieldpath="/modules/mod_royalslider/fields"></fields>

the error gone and the field disply as a text field.

You forgot to specify models in the addfieldpath

<fields name="params">
   <fieldset name="basic" addfieldpath="/modules/mod_royalslider/models/fields">

       <field name="title" type="City" label="anythging" description=""   />

   </fieldset>
</fields>

Just put the fields folder inside the models folder and put the path like above.