There is an autosave feature, however depending on your settings, the browser might disposed the savefiles after PC restart or just block saving altogether.
Viewing post in No auto save? Really?
I play a ton of incremental games, here on itch and other places and I've never had an issue with save games not being... well, saved. There was no slow down (I had less than 300 soldiers), the game simply froze completely. Refreshing the window (Not shutting the browser or turning off PC) resulted in a fresh start with no save.
All I can say is that I'm sorry for your experience.
We tried to make it work as good as we could and auto-save should work. It happens every 60s and if the savefile got corrupted there would be an information about that. If you got a fresh start then it means the browser just magically removed the save.
Maybe there is an unknown bug. We'll keep an eye out. But I think it's more likely to be related to the webGL build since nobody from our community experienced this in the past few weeks of playtesting. We wanted to make the game as accessible as possible which is why there is a web build, but a preferred way is definitely to either play it on Steam or download a standalone from itch.