CGCookie - Developing a Tower Defense Game

Mixkit: Known for its user-friendly interface, Mixkit provides a vast collection of free video assets, including motion graphics and video templates. While they aren't directly from Videohive, they offer a comparable quality that can be integrated into your projects seamlessly.
Videezy: Videezy offers a variety of free and paid video assets, including After Effects templates. Their extensive library is perfect for those looking to enhance their videos without breaking the bank.
Motion Elements: While Motion Elements primarily focuses on premium content, they regularly feature free downloads that include video templates similar to those found on Videohive. Their search interface allows you to filter out the free options easily.
Editor’s Depot: This site provides free editing tools and templates, ensuring you have access to some elements similar to Videohive offerings. Though the selection might not be as vast, it’s a valuable resource for cost-effective production.
Motion Array: With a solid reputation, Motion Array occasionally offers free downloads that replicate some Videohive elements. Subscribing to their newsletter might help you get alerts about any new freebies available.

https://cgcookie.com/courses/developing-a-tower-defense-game

In this course you'll learn how to build a scifi tower defense game in Unity. This course will include a series of 3d game assets, audio clips, scripts and 2D elements to build a tower defense game from start to finish including a main menu, splash screen and final build of the game. This is an intermediate course and aimed at those who have a basic understanding of C# and Unity.
The game you'll be building is called Portalethium and is our take on a tower defense game. This will involve 3 enemies and 3 "towers" which will be turrets that fire on the enemies. The enemies will be spawning in via a portal and trying to get into your portal. Your goal is to defend your portal by placing down turrets that will attack the enemies. This will be a "wave based" game that will end when the friendly portal has been destroyed.