A minor miscalculation...
If only I'd saved up five more!
I feel like with most of your games I end up saying "I'm not very good at this type of game even without the zombies" but this is an exception! I'm much better when there are zombies to splat, they give me points which mean I don't need to be very good at these kinds of puzzles (which I'm not.) I finished the first 3x3 grid thanks to a single zombie smashing into exactly the one light I needed it to, and after that I was able to pay Zombits to finish up the larger levels, and I would have finished the 15x15 if I was better at counting. 🙃
I never felt much danger of my heart being eaten, since the zombies struggle to break through one block at a time. Even when the hordes arrive, it's generally simple enough to switch off a path for them to come in and then immediately splat them. Interestingly, the larger the puzzle is the easier the defenses become.
I suspect that if I was actually good at these types of puzzles I might get more annoyed with zombies coming to ruin things. Admittedly there were a few times where I would have been able to finish the board with a few of the Small Light item, but the horde showed up exactly on time to prevent that. That was still fine, since more zombies means more Zombits, and a Clear Board followed by buying Toggle Row is still cheaper than patching the tiles I couldn't figure out how to light legitimately.
It would be really nice if there were keybinds to quickly buy a shop item, and also to be able to cancel a purchase; more than once I accidentally clicked on Toggle Column instead of Toggle Row (or vice versa) and was stuck ruining my own board with the upgrade I hadn't meant to buy. That's definitely something of a skill issue on my part, though...
Anyway that's enough rambling... I think this is my favourite of the 'Just Add Zoms' games so far, great job!