Go to "All games...", inside My Shelf (left column of this page). Click on "Imperium Romanum" and look at the rightmost column of the page. Below the blue "download" button there must be a link to the manual.
If it is not available where rcantora pointed out (not all older games come with the manual), you can always download from here: 'http://replacementdocs.com/download.php?view.6734'
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