Crimson Dragon: Side Story is currently unavailable to purchase from the Windows Phone store. An article on Windows Phone Central discusses the reason for this, namely that Crimson Dragon: Side Story and a number of other games are incompatible with Windows Phone 8. Apparently the underlying XNA architecture works differently in Windows Phone 7 and 8, causing some games designed for Windows Phone 7 not to function on the latest version of the operating system. When questioned about the issue, Microsoft responded with the following statement:
The Windows Phone 8 platform was designed to run existing 7x apps, but some may require a degree of optimization given the new hardware and software capabilities. We are encouraging all developers to test their apps with our new SDK to help ensure that apps and games run great and take full advantage of new features. We expect the catalog of existing app titles on new devices to grow steadily over the coming weeks and months.
It seems likely that Crimson Dragon: Side Story will return to the Windows Phone store at some point, but it’s unfortunate that the upgrade process was handled so poorly by Microsoft, with some developers having not been acknowledged that their games would be removed. We’ll post another update here when the game is made available again.