How to assign grades in a game development course?
I would think that one technique you might choose to use in this case would be to establish what your learning objectives are for the assignment and then construct a simple rubric for evaluating each project.  You will likely have some areas that will be present for each assignment (e.g., documentation, code clarity, compiler warnings, possibly even a baseline score for a runable attempt, etc.). However, you will probably have a few aspects that are required to be present for each assignment (e.g., sprite usage, pathfinding, narrative) which you can then assign points for.

For example you could use something like the following:

Narrative


5 points - had a clear and easily understood narrative
3 points - had a narrative, but some portions were confusing or difficult to pick up on
1 point - an attempt was made at a narrative, but it was generally insufficient
0 points - no attempt at a narrative


Pathfinding


5 points - no (or almost no) issues with pathfinding
3 points - pathfinding mostly working, sometimes units get blocked behind obstacles/jump through obstacles/etc.
1 point - pathfinding is clearly deficient


For project based courses or assignments, I find that a rubric like this will greatly simplify my life when grading and lets me focus on giving constructive feedback over trying to decide if this is an 82% vs 85%.

Alternate approach

I have also had project based courses where the grading was not based on the artifact, but on a report and presentation/demo that the student teams had to submit. I also used rubrics for scoring those, again greatly simplifying the process and letting me focus on feedback over points.