Send your units out to the castle. Sacrifice them for cash or earn money upon arrival. Buy upgrades for your units. It's basically reverse Tower Defence.

I created all assets except for the coin-sound and the cash-register sound.

Here are some work-in-progress gifs: https://imgur.com/a/l6YkwdI

Edit: I was able to fix the error. The Windows-version should run in Windows 10 at least. I could not get the HTML5-version to work.