Ok, so here is how I did it. Again, I used left-over yarn, combined it in a bulky thread and crocheted using a size 12 crochet hook. I also used a hoop made from a flexible plastic rod that I rescued from an old window cover. I started the beginning row by crocheting single crochet stitches around the hoop. Slip stitch to close the circle and then worked in circles using double crochet only. Because the hoop was a bit large I decreased from time to time (3 stitches every 2 or 3 rows). When I was satisfied with the length of the cave I stopped and left the end of the cave open. Then I asked my cats to test it :P