The idea for this gameplay feature comes from a mod called MetaGameV2 Mod Version 2 (22/Nov/16) made by Eraser, who wrote:
[...] this mod is more than a game it's a pool of concepts and ideas for the main game, using existing resources.
The basic idea is to give the Spartan civ a strong weapon to defend against cavalry attacks. The spike deals damage against cavalry units that get too close to it.
Here are some short videos:
- Attack from behind has no effect**
- Attack from the front causes damage **
I only chose the Spartans to make the civ more unique. Could also be extended to other civs.