All enemy sprites fall through platforms, rather than staying on them, even though I have physics enabled for enemies.
Do you have a collider managing interactions between the enemies and the platforms?
Adapt this line from your code:
game.physics.arcade.collide(player, platforms);
Also, please check out other threads and Phaser examples, and add code snippets to your question to save time and help others to help you 
The link below has a lot of examples which will help you understand some intricacies of Phaser more practically than trying to interpret the documentation:
https://labs.phaser.io/index.html?dir=&q=