Tower defense games are generally a hard sell to me because there is a core design problem to solve with this genre: the death spiral. At any point in your tower planning and wave management, it's possible to make a move that has doomed you to failure well before you've actually hit a game over screen. Different games have different ways of dealing with this issue, with my favorites virtually eliminating it. Isle of Arrows comes nowhere close to solving the death spiral problem, but its design is still fascinating enough that I keep picking it up to try and master it.
Tiles and towers
The primary twist Isle of Arrows adds to its brand of tower defense is a system where you construct the level you are playing on as you clear waves. When you aren't watching your fortifications shoot arrows, cannon balls, and more at swarms of enemies, you are offered cards that you can place on the level to add additional features it.
Sometimes these cards are new towers or path cards you can use to extend how far enemies need to go before reaching your defense point. Beyond that, the cards can also be less direct features like a garden that provides a small currency bonus or "flag" tiles that build out more tiles on the island you're playing on.
This wrinkle gives Isle of Arrows a whole second layer of strategic depth. In addition to trying to optimize a path that only ends in death, you have to make room for it. The only problem is, this game isn't kind about giving you what you need so much as serving up a limited selection of things you have to try and make work.
You can make the enemy path as long as you want, for example, but unless you manage your currency to be pulling down and placing towers along it you will eventually get overwhelmed by the enemy hordes. At the same time, there are a lot of bonus tiles you can add to your island, but doing so cuts down on the real estate you have to work with, and if you want to accelerate any of those plans you have to manage your currency efficiently. It's a lot of juggle at once, but Isle of Arrows gives you all the time you need between waves to ponder your plans.
Isle of Arrows has a few different modes to engage with (including a daily challenge), but they all boil down to essentially the same idea: Survive through as many waves as possible. The game's difficulty curve practically ensures you can't do this on your first try on the game's campaign mode, though (presumably) gives you all the tools you need in the daily challenge. It's also worth mentioning there's a third "gauntlet" mode that unlocks once you've cleared a campaign that also has all cards unlocked by default.
I say failure is almost certain at first because Isle of Arrows definitely has a problem with death spiraling which is exacerbated by the fact the card unlock system in the campaign mode, which is unambiguously this game's primary mode. Access to every other mode is tied to progress in the campaign, so not only will your first few forays into the game likely put you in situations where you just know you don't have enough coins to cycle cards and get your defenses up to snuff, but after you fail you get shown some tools that probably will make it easier to get further next time. All this can be demoralizing, but I somehow find the experience compelling enough to keep diving back in to see if I finally have the player knowledge and cards to clear the next hurdle.
The bottom line
Isle of Arrows has all of the design features that would prime me to be turned off by it, but its unique take on tower defense charms me nonetheless. Anyone looking for a deeply strategic tower defense game offering something genuinely new in terms of concept will be more than satisfied with it.