Thank you for playing!
I appreciate that you were impressed not only by the audiovisual side of the game, but also by the mechanics of it! The levels, although seem massive, aren't really all that resource demanding. They're the size of 500 square meters, and are built of static models, which are also partially hidden by the fog.
I think that the problem in the round 4 might have been caused both by the low health, and the night time. There is a day and night cycle in this game, comprised of repeating sequence of 2 rounds during the day, then one round at evening, and one round at night. That being said, if you don't have any light sources with you on round 4/8/12/etc, you won't be able to see much. However, when your health is below 50%, the saturation of the screen will decrease, and the contrast will rise. That is problematic, because even if you have some kind of light source with you, due to the high contrast you still might not be able to see anything, which has to be fixed.
As to the source code, you need to download the master, in a .zip file. Upon downloading, you need to extract files, and then in the Swift-Survivors-Master folder, you need to unzip the Source Code.zip. When I've tried to do that, it did manage to get the source code, so that should work!