A somewhat impulse play Medieval Defenders appeared on my Steam Suggestions and I was appalled by the greed involved.
Created by Creobit and published by 8floor Medieval Defenders is a very basic free to play tower defence game that relies on its cute graphics and outright devious limitations, to make players purchase microtransactions.
Elixirs are the ingame item used for enhancing turrets and spells and can be earned in small amounts each level, however in later levels the best tactical turret locations also cost Elixirs to use.
To give a full example of the game I've played through the first 10 of the 40 levels.
Steam Store Page: http://store.steampowered.com/app/522090/
