The shells of pulmonate land snails must offer some protection against their predators. After all, the shell is a hard solid structure that in most cases envelopes the entire snail and the aperture could be blocked by an epiphragm or by various folds and lamellae (here is an example).
However, large predators of snails, for example, mammals and birds, can easily break snail shells. The picture above shows some shells of Eobania vermiculata from Istanbul, Turkey. I found all of these shells at one location. All, except the one on the lower right hand corner, had similar breakage patterns indicating that the cause of breakage was the same in each case. The most likely predator was probably some rodent, a mouse or a rat.
Smaller predators, on the other hand, including carnivorous snails, enter the shell either through the aperture or by drilling thru it. The exact function of the folds and lamellae that commonly block the apertures of many species is actually not clear.
The picture below is of a shell of Albinaria caerulea (from near Ephesus, Turkey) with 2 bore holes that had been made by a larva of a Drilus beetle. Drilus larvae are common predators of land snails in Europe, Africa and the Middle East. But only in Greece, Turkey and the Middle East do they bore thru snail shells.