People love the Castaway series, and there's a lot to love. That plucky protagonist with his spiky hair. His adorable pets. The funky tropical locale and seafaring music. Tower defense fans looked on in envy. Well envy no more, because Likwid has come to your rescue with Castaway Island TD.
Borrowing some controls from the action rpg originals, you actually have to move your character around with the [WASD] or [arrow] keys and interact with potential tower slots through the [spacebar], using the mouse to select which option you want. You also have to move around yourself to collect the coins that enemies leave behind when they die. In addition to attacking enemies with towers, you can also lay traps for them, either by clicking on the desired trap in the lower right or by hitting hotkeys  through . Traps drop right where you are. The game includes the signature pet feature of the earlier games. You permanently pick one of three pets to help you, and they act automatically. The one big drawback is if you're hunting for a "speed up" button. There isn't one.
This is no mere rushed attempt to exploit a franchise by exporting it to another genre all slapdash like. The makers clearly put some thought into making this its own game, and they understand what the genre's fans like. So you can stand proud, tower defense fans. You have a Castaway game of your very own!