Fighting for my garden

I had a look at my garden (very small little L shape)

Picture from Summer - now it looks too sad for proper photo.

 and discovered that half of my plants are getting eaten! My little herb garden is mostly decimated. Thyme almost gone, sage has only leaves on top and oregano is none existent. In winter? You know we have really bad problem with slugs and snails that I had to give up some of my home growing. My onions, leaks and carrots didn't survive. Others like beans and strawberries I have been fighting for all summer.

So I did some research and bought slug and snails resistant plants and these are getting eaten now!
I think that the problem is winter caterpillars. I have saw a few little green ones. I am trying not to use any "chemicals" in our garden but sometimes there is no choice. I have found this :

Py bug spray is natural insecticide containing pyrethrum (from plant) so I hope I will get rid of the green monsters and not do any "toxic" damage to my garden