Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
Slay is what happens if you take a strategy game along the lines of Civ or Wesnoth or Homm3 and remove features until the absolute minimum is left.
Which brings me to one of my pet-peeves. A lot of people, including game reviewers, say 'depth' when they mean 'breadth'. "This game only has one button? Where's the depth?" Breadth usually comes from adding verbs, things to choose from, things that make nice bullet points in your marketing materials; depth comes when the small number of verbs you have, combined, create their own variation. Go is a game with almost no breadth - there's only one type of unit - and insane depth. Your typical fighting game with a hundred moves, only one of which you use because it's an exploit, is the opposite.
Breadth can be the enemy of depth. Anytime you give a player a new option to choose from you run the risk that you've created an exploit that makes all your other choices meaningless.
Slay is an old game - been around since Treyarch was half-a-dozen guys in a condo in Venice - and I didn't think too much of it back then, because it seemed too easy. You had to really look to find one of the randomly generated countries that was challenging. Well, maybe back then I had the AI set on a low level and didn't realize it, or maybe I've gotten stupider, or maybe Sean O'Connor has been working on the AI over the last ten years, because now it's pretty tough to beat. I just managed to claw my way to victory on one of the small islands, and there's a whole lot left.
That said, the AI isn't on par with your typical chess AI. You can leave a weak unit out where a stronger unit can get him, or lower your defenses, as bait, and then cut the strong unit's support. Which is terribly satisfying! If the AI was better, the game might be less fun...almost like the AI in a stealth game.