How do I run it? I get that LD36.jar, so what do i do with it?

You should be able to run the LD36.jar

Make sure you're running the latest build of java too!

This game isn't particularly interesting - but I'm not sure if that is a judgement on the game or not.

Sadly I just don't think there's enough there to properly give it real feedback.

It certainly is a tower defense game, so there is a strong base to build off of, and there are a lot of places you can go with it.

I'll just list things that definitely need to happen to make it a more complete game:
- There needs to be more than one tower
- There needs to be more than one kind of enemy
- There needs to be variation in the waves
- Tower upgrades (or similar game mechanic)
Of course, all of these things are obvious, as they are central mechanics to a tower defense game - so I guess I'm not really helping you. Sorry. :(

Other feedback I have:
- The cannons do not always render correctly when they rotate - I would suggest looking into making sure your image libraries work correctly. ;)
- I don't think you are allowed to use the textures from Minecraft - I guess there's no explicit rule against it, but the rules do suggest to only use assets that you have the legal right to use, and I'm not sure if you do... (I, of course, wouldn't really worry about it too much, but if you want to work on this game in the future, be sure to create new images)

Finally, please do not load images from the external Rsc folder - if I cd out one directory higher, and run the program from there, the files will not be found. Some Linux systems run Java programs like this by default (that is, the working directory is not the location of the jarfile) so your program is not cross-platform.

What you should do is package the resources into the jarfile - this is pretty simple to do, simply change out your image loading to use [Main class].class.getResource() in the ImageIO.load() statement.