One of the key differences between Rift Riff and other tower defense games is that Juice, the build and upgrade resource in Rift Riff, is generated during waves and not simply granted to you after you complete a wave. Story-wise, the juice-generating towers are actually the very reason the enemies are attacking you... but I won't be going into the game's lore here. Because juice is generated during waves, you'll find yourself going into the wave with one plan but restrategize into another: more and more monsters enter the battlefield, more juice flows in, and you have more and more build options.
How much juice is generated each wave depends on how many juice-generating towers you have. Those kinds of towers can only be built on these different-looking tower bases that have a bit of energy flowing out of them. We're currently calling that class of towers 'Defensive towers' but that name may change before the game comes out.
Your Basecamp generates juice, as well as the towers with the fitting name 'Juicers'. There's a few more towers that generate juice, but we're still designing and improving those. One of those is an armored well that doubles as a bunker with a lot of health, but doesn't generate as much juice as its alternatives as a result.
One feature in Rift Riff that feels really good is that destroying towers fully refunds the costs of both building and upgrading that tower. This means that after barely surviving a wave, you can refund a bunch of towers and restrategize your defense based on the incoming enemies.
That should explain the main build resource in Rift Riff! If you have any questions, drop 'm in the comments.