The yellow squares make up the path that the enemy (red square) follows, with some arrow squares to indicate which direction it's heading. The enemy is actually just moving in a constant direction and whenever is passes over an arrow tile, that updates it's direction. When it goes off the end of the path it reappears at the start. So in this prototype there is just one enemy who continually loops around the path.
Along the bottom you can see the names of five different instruments. Clicking on one of these will cause a coloured square with a circle around it to track your mouse while still conforming to the grid-based nature of the game. The colour of the square indicates which instrument it is and the circle shows it's range. When you select an instrument it also shows the range of all other instruments of that kind, allowing you to see which areas of the map need more coverage. In the screenshot above the Lead instrument has been picked, highlighting the range of all other Lead instruments.
Whenever an enemy is within range of any instance of a particular instrument, that instrument's part of the backing music will turn on. It turns off again if the enemy goes out of range of all instances of that type of instrument.
It was a bit awkward to get smoothly looping music tracks to play in Flash since it doesn't natively support any embedded music formats other than MP3 (at least at the time it didn't, it may do now). You could stream in WAV files from a local directory but you couldn't compile them into the webpage-embeddable SWF file. The problem with MP3s is that the format requires a small amount of silence at the end of a track, thus making a smooth loop impossible. There are several hacks to get around this but I found the easiest work-around was to use Bottema's as3wavsound library. This allows you to embed WAV files which can be made to loop smoothly.
The main problem with this project is that it requires separate audio files for each track in the song. This takes up a lot more memory than the normal track would and is also difficult to acquire unless you're the artist making the song. The song used in the prototype is an example piece titled "Surrender-Main". It's taken from the Shorties directory of version 0.4.14 of the Linux Multi Media Studio. Since it was an example piece included with music creation software I had access to each of the individual tracks so was able to export them separately. I could find no mention of the original artist and claim no ownership over the song.
Without the talent to create a decent number of tracks myself nor any musician friends willing to spend hours doing so in my stead, this fun little side project came to an end. There are design considerations to be made to ensure the abrupt inclusion and exclusion of music tracks isn't annoying, and it would take a talented musician to create a series of tracks which still sounded good with any possible combination of individual tracks, but I think this could make for an interesting game with enough effort.