I saw a video on TikTok the other day of someone using thorny branches (like from a rose bush) to border the planter to stop slugs.I agree with @Moongirl69 , topping up the soil will help getting a bigger harvest. I always leave mine in until the foliage has died, no harm in leaving them in since it will be too cold by then anyway for them to start growing.
Also can I just rant about slugs? bleeping hate them, they've made a mess of my spinach and radishes. I don't want to use poison but at this rate I might not have any more options. Ffs.
I was going to try it but I haven’t had any problems with slugs yet.