thats just a background decoration

personally i usually turn them off with the F1 key (i think that's the right key)


anyway if you are stuck you may have missed an illusionary wall ora key
(if the options were active on dungeon creation),
or a hidden switch, given the wall texture in your screenshot it should not be too hard to notice

also i believe there is a rare chance that the game could construct a level that is unwinnable

oh and another way to the next floor is jumping into a pittrap (if traps were turned on in dungeon creation)



i hope this helps ya
happy dungeoning