Click and drag to draw a line. Enemy lines come in from the edges and yours stop them, but every line you draw spends power you do not have much of, so the question is never whether to block something, it is whether this one is worth it. The base in the middle grows towards a target radius and the level ends when it gets there.
Coins buy upgrades between levels and powerups drop mid level: slower enemies, unlimited power for a few seconds, a growth boost, a scatter of coins. The scoring runs backwards, so fewer lines and less time is a better result, which quietly turns a defence game into an efficiency puzzle. It saves after each level, and the leaderboard in your own head is the one that matters.