All the validation rules you defined in a validation.xml file will apply to both client-side and server-side validation. Even when client-side validation is turned on,
Struts Still does Server-side validation. There's nothing you have to do to turn that on. It's done automatically. All you have to do is make sure your ActionForm extends ValidatorForm and add an <html:errors /> tag to your JSP so that there is a place for server-side errors to be displayed.
Note that when client-side validation is enabled, server-side validation is really just a backup plan, as the client-side validation should pick up any errors before the request is even submitted to the server. However, because a user can disable JavaScript on the browser, it's always safer to do server-side validation also just in case.