Upgrading towers, sounds, and classes in lua

So since the last post, I've added a few things to the tower defense game...

(1) I added menus! I actually got bored one day and wasn't sure what to do so I figured I'd get it out of the way. The main menu includes a campaign button that takes you through each level one after each other, a level selection button where you can choose just to play one level and then go back to the menu, and, of course, a quit button.

(2) Upgrading towers! Before, I just had three towers, and honestly it made gameplay kind of boring... there was no sense of "progression" or getting better. The way you upgrade a tower (as of now) is to just drag one tower onto another tower (with some extra cost) and it combines their damage. So if I combine a red and green I would now have a yellow tower (additive color model). Now this yellow tower can damage both red and green components. I also made it so when you upgrade a tower, it can target one more enemy (with the max being 3). As of now, you can upgrade towers to max level on every level, but I want to make it so you get another upgrade per level, so by the final levels you have all of them. This would give some sense of progression.

(3) Sounds... So I had an interesting idea for sounds. I've taken a few music theory courses, and I know some cool sounding chords. So my idea was to have each tower play a note when it attacks. When all the towers are attacking at once, you get this cool sounding chord. Right now I only have 4 notes for 4 towers (I need to put sounds in for 3 more towers). I still need to work on my execution of the sounds though (you can hear how it sounds in the video below)

(4) So another thing I did was to refactor my code a lot to use metatables (mimic classes). I found out about these things in lua called metatables. You can specify a metatable for a certain table and if you call a function that the table doesn't have, it will try to call the function with the metatable. For instance: say I have an Enemy table (enemy class) with an update and draw function in it. Now I want to make an "instance" of this Enemy, so I make a new table e to store the data, and set e's metatable to Enemy. Now I haven't set the function update or draw in e. So normally if I called e.draw or e.update there would be an error. Now that e has Enemy as a metatable, calling e.update is equivalent to calling Enemy.update and it's the same for A.draw. It makes the code a lot more readable, and actually uses less memory than the way I was doing it before (I'm not 100% positive about that). My reasoning for the memory is: Every time I was creating a new enemy, I would create a table with each function in it. So each enemy had it's own instance of the update function for instance. Now I just create one function in the metatable, and each instance calls that function. I'd imagine this reduces memory but I'm not sure.

Hope you found this interesting!