I like idle games with interesting/compelling premises, so the concept of a really stubborn king who's trying to brute force his way towards the ocean is hilarious. Just like one of the commenters already said however, the jump from having to grind 1,000 coins to 10,000 coins just to build the third floor of the castle is too steep for the early game, especially since there's no way to move units around (either to move them into empty slots or return back to the inventory). There's also no penalty/effect for when the countdown runs out, so it can make having to repeatedly press the "fight" button to continue the grind very tedious.
The visuals are solid, and there's a lot of potential for an idle game whose main mechanic is a combination of grid-based inventory management and tower defense. Having the walls "fight back" or do other effects to slow down progress could also be a good starting point for further prestige and ascension mechanics.