It's actually a really interesting little strategy game, although it's not presented that well. Having only one unit at a time turns out to be a pretty substantial twist on the usual games of this genre (I can't remember what the genre's called but I've played a couple like this.) I think with some expansion it could be a pretty fun either single player campaign or mini multiplayer game.
That said there are some really weird balance issues. I think the biggest miss is that units don't refund in any way when you recall them. With how slow gold is generated that means that to swap a unit out costs like a whole twenty seconds of resource collection. This is especially a problem since retreating units under your castle to shield them from your opponent's units is so good, and it's easy to accidentally recall them. I think an interesting twist would be to make it so you have a "bench" that you can store one of each unit on when you recall it. Swapping units around seems like where a lot of the strategy could come from if developed more, so I think it shouldn't be penalized as much.
Even though the presentation and mechanics explanation is not great, I found my self having fun learning how the game worked and it took me a good few attempts to make any progress.