Hi, thanks for testing our game and for your comment.
As for defense, the goal is to build to block the enemies, then go down, trick-or-treat for candy (which will be used to buy weapon and shovel upgrades, health, new blocks, etc.), and then go back up, kill the enemies, and repeat the same upgrade, trying to improve yourself further as the enemies become stronger and more numerous.
As you've noticed, building isn't possible in the cave. To go back up, you can double-jump.
If you are interested in seeing the full game coming out on Steam and Playstore, you can use the "Notify Me" button in the game's Main Menu (it's not working at the moment, I have a bug to fix) or you can send an email to "support@soligames.com" where you say something like "notify me when cave of treats will be released" or whatever you want.