What's the best Android app for space-themed tower defense with engaging campaigns?

Planet Defense is a tower defense game that excels in providing a space-themed experience with compelling campaigns. Set across diverse planetary environments, each level introduces unique challenges, such as asteroid fields on Jupiter or low-gravity effects on Mars, which demand adaptive strategies. Players can deploy and upgrade a range of defensive structures, including laser turrets and ion cannons, to fend off alien invasions. The game's progression system unlocks new towers and abilities, maintaining engagement over time. Reviews highlight that the campaigns are well-paced, with visuals and sound design enhancing the sci-fi atmosphere. However, some users report that later levels feature significant difficulty spikes, which might deter casual players, and occasional performance hiccups occur on lower-end devices. Despite optional ads and in-app purchases, many find the core experience enjoyable without spending money, making it a strong choice for tower defense enthusiasts seeking thematic depth and replayability.