Zend中的只读路由参数？

Here's my example route:

routes.subcategory.type = "Router_Route_Subcat"
routes.subcategory.route = "([^/]+)/([^/]+)"
routes.subcategory.defaults.controller = "Category"
routes.subcategory.defaults.action = "sub-category"
routes.subcategory.map.1 = "category"
routes.subcategory.map.2 = "subcategory"
routes.subcategory.reverse = "%s/%s"

now I would like to add a parameter to the request object, which is easy enough by doing:

routes.subcategory.defaults.year = "2010"

In the action I can then do:

getRequest()->getParam('year');

But I want to be 100% sure that this 'year' parameter can not be set or overwritten from the outside world, not by adding query parameters to the URL, nor by posting the year parameter to the URL.

If I try it out I can't seem to change it, but can I be sure?