Defender's Quest: Valley of the Forgotten, a Tower-Defense/RPG Hybrid, has added Workshop integration for modding a variety of aspects of the game.
Modding Defender's Quest will let you change every detail about the game: graphics, story, audio, enemies, character classes, and whatever else you can think up. Now you can easily find, install, and share mods with just a few clicks.
The developers have put together a great Steam Guide for getting started with their modding system, which you can find here. You'll also find a couple fine examples of their mod tools in action with a couple mods already in the Defender's Quest Workshop. One created by community member "Melon" and another created by the developers themselves.
Plus, save an incredible 66% when you buy the game before July 1st.