USPTO Trademark
On June 3, 2008 - COM2US Corporation was awarded the trademark for the term "Tower Defense", filed on June 13, 2007 - serial number 3442002. The corporation has been reported to start enforcing the trademark; specifically on the iOS platform as a number of small or independent developers are now receiving messages from Apple citing trademark violation. Adding the phrase "Tower Defense" (in capital letters) to the description of an app submission to iTunesConnect and the app store automatically throws up a warning informing you that your submission is likely to be rejected for use of the term, however writing the phrase in lower case is still acceptable as "tower defense" is a valid description of a game style. The trademark only applies to the capitalized Tower Defense.
Read more about this topic: Tower Defense