The rugged cavern section is covered without major trauma, then the party finds a more structured (aka „square shaped“) section of storerooms and are dutifully explored (see below, when there are a series of rooms to investigate it is always interesting [fun for the DM at least] to see „who“ goes [or is nominated to go] first into the ’next room‘, a bit of reluctant „turn taking“ is always FUN (karma inducing moments) – as in who’s time is it for their luck to run out and spring a trap): 

An interesting find. Rinse and repeat. Steps leading down, especially when they are narrow (5′ wide which means single abreast), which is always a „bad sign“. The classic 10′ by 10′ cube awaits at the bottom with the classic choice of two doors (see below, „Wasn’t this why we brought a thief along, to go first?“ says the fighter to the magic-user):

Behind one of the doors .. is a guardroom (what else would you expect) garrisoned by drunken brigands who fight at disadvantage (see below, one sleep spell later and they are tied and bound up and ready to sing like canaries): 

The party advances to meet an important bad NPC (magic user) whom escapes .. with the expense of his beloved familiar — which all adds rich narrative texture to the adventure which is good stuff! Don’t worry they meet up with him later and he has a score to settle (he was attached to that rat the party killed)!