to add further to Cyangmou's great example, if you should wish to push it one step further, and reinforce the depth with both colour and foreshortening, like most things in art you can enhance certain aspects by exaggerating them, use the above method as a base to get the form in the aspect, then add some exaggeration and colour cues to push the depth and 3-Dimensionality of the object.
another thing to think about is symmetry, it can be used to make things faster, and it also has some benefits in making things look cheap and less organic, but there are benefits to be had from it, and if you fuse the two methods sometimes the end result can be better than rendering both sides on your own, for example working it until it looks just right proportionately with symmetry, and then adding asymmetry afterwards, things that don't work with pure symmetry are side lighting and every type of secondary lighting besides back lighting (or it becomes at least triple lighting(an identical light source from both angles, which in all honestly does not look very natural at all))
Edit to illustrate some things:
-you'll notice a side effect of the projection is that the scale changes
-blue and cyan on the base is to indicate a possible approach to emphasising the highest points to reinforce depth.
-heads (poorly) illustrated the effect of tilt, which makes you consider another factor 'what angle are certain features at?'
-A is an edit using some methods to reinforce the depth a bit. B,C,D are all variations on symmetry which some of which have a more cohesive aesthetic, and could be used to base an asymmetric version off. of which factors worth paying the most attention to are: off centre lighting and possible secondary side light source, of course in the context of a game secondary lightsources increase the workload, to do convincingly you can't just simply mirror the horizontal stands and walk cycles, unless you cheat and omit any secondary light source from the side views, which will reduce the consistency of the sprites themselves.but just remember there are various ways to convey depth not just implied foreshortening with colour.
yes to some degree it very much dependant on the style of your graphics and the level of depth you wish to incorporate.
I will however add that most games that use rather flat projection (which is a necessary in tile based games, there is no escaping 2D, only hiding it and doing your best to imply a 3 dimension), try to avoid the orthographic flatness in the games sprites and objects even more so by adding more depth than simply getting the overhead angle exactly right. and it may represent what you want, but it won't convey it's height from the terrain as much as it could, often it is much more effective to break this in various parts
depth of course can and should be added to everything even grass is taller than the ground plane and good games even add depth to that, unless you are doing flat ground intentionally.
rather rushed and not entirely conclusive example:
on the right are some quick examples of how even with the right shape you can make things look flat without taking the right approach (the noticeably flatter attempts are marked in yellow)
value, colour, even shadows can be employed to make the depth more noticeable.
well probably laden with mistakes I'm having trouble managing my time and am rather busy, so feel free to chime in if you believe anything is incredibly wrong or not elaborated/articulated correctly, as the subject is reasonably advanced and there is no real perfect formula or method. but awareness is one step in the right direction
