This is a paid asset, but now you can download Tower Defense Toolkit Free.
Detail this asset from Unity Store: Original Link
Tower Defense Toolkit 4 (TDTK-4) v4.0.5 f2 (Latest version)
Tower Defense Toolkit 4 (TDTK-4) v4.0.5 f1
Tower Defense Toolkit (TDTK) v3.1.1 f3
“If possible, please buy the package to support the developer”
Official Website – Link
Unity Support Forum – Link
Quick Video Overview – Link
WebDemo – Link
TDTK is a C# coding framework for the easy construction of TD games of any kind. It comes with a bundle of scripts that are flexible and configurable to fit a variety of TD gameplay scenarios. It’s mobile compatible.
The toolkit is designed to be used with custom models and art assets. You can easily integrate your own art assets to make your unique TD game.
*Please note that fps mode from previous version of TDTK has been dropped.
 – Free Download){kind=link}