You've got a few options, but really, the two which make the most sense in regard to an isometric-projection (45-degree angles), are tiles and full-fledged 3D with a static camera (look at XCOM, StarCraft II, Diablo III).
Of course, you can also do without tiles if you're, say, making a game like TMNT2 - The Arcade Game or River City Ransom, and it's an old-fashioned hallway brawler, where in a modern version, rather than tiles, you could use one large hand-drawn background, if you wanted, (plus parallax images) and then create collision-nodes in-place, on the map.
Also, creating a map using tiles becomes vastly more simple, as soon as a map-editor is written.
What is the client actually trying to accomplish, here, and on what scale/budget/timeframe?