You, a bumbling cloaked wizardy figure, tumble out of a wormhole into a new world. You jog a short distance to the spot you can install your extractor—which is a tower that sucks juice out of the land. Your initial juice budget lets you buy a few defensive towers well, which you place along the path you just followed, which also happens to be the path shadowy blob monsters will follow when they pour out of space to come for your extractor.
One of the things that makes Rift Riff special is that, as the Steam page puts it, this is a "remarkably forgiving" game. You can refund a tower for its full value, at the start of each wave you get back any towers that fell, and if you die then you respawn at the start of that wave rather than having to redo the whole level, and you respawn with all the towers you'd built in place so you can refund and edit your layout rather than having to reconstruct it from scratch. Also you can collect flowers for bonus juice.
It's just real pleasant to experience. The music's delightful, and so is the way your stumblebum hero goofs into and out of portals. Rift Riff's co-creator was Adriaan de Jongh, who previously worked on the equally delightful Hidden Folks—a hidden object game that couldn't be more different genre-wise, yet when the protagonist of Rift Riff bumbles around making mouth sounds I do get a little of that Hidden Folks vibe.
Rift Riff's [[link]] closest cousin in the tower defense genre is probably Thronefall, and conveniently you can get the two games in a [[link]] to satisfy your desire for lo-fi tower defense. Or you can just find Rift Riff on its own where it's on sale for like $US6.