Sorry it isn't clearer, but you only control the game with the mouse.
At the beginning of the game, buy a tile from the shop that says that it spawns a unit, always on the left side of the shop, and then buy whatever else you want/can from the shop, and press "GO!".
Then drag those tiles into the castle, with the spawner tile showing where your units will walk, and place the other tiles on that path to buff them. When you run out of hours, or you press the "to dawn" button, combat will start. During combat you can use the tools to move units, hurt units, or buff units, and you go to the shop again after the fight.
Rinse, repeat, until you beat the game or lose. 👍