publishing – I want to make a video game based off of Dungeons and Dragons. How much am I legally permitted to use?

I want to make a video game based off of Dungeons and Dragons. It’ll be a top-down RPG with similar battle and class mechanics. But I’ve been planning out more, and seeing how much I’m copying. How much am I legally permitted to use? I am awayer of the SRD, but don’t understand it fully. Is it like a book of what I’m allowed to use?