At some point prior to descending to the second level of the crypts, Keep on the Shadowfell expects players to take a trip back to town. But before they do, let's look at level one's secret chamber.
The Hidden Armoury is set into a corner of the Crypt of Shadows. It's cordoned off from the rest of the level by an illusory wall.
Illusory walls are old-school. In previous editions, between the dungeons and the player-run wizards, it seemed like every second thing was illusory. Still, they're stupid. It's a strange world indeed where it's easier to set up a permanent magical field than to install a door with a lock.
The reason Keep on the Shadowfell is doing it is to teach players about the difference between "line of sight" and "line of effect". While you need line of sight to be aware of an enemy's location, you need line of effect to be able to actually attack it. A pane of glass, for example, might break effect but not sight. Here we have the opposite effect, with the illusory wall permitting arrows and such like to fly through it while obscuring vision.
On the other side of the illusory wall are some more zombies, and, once they're disposed of, a riddle. A magical plaque poses players a cryptic paragraph, and if players solve the puzzle correctly, someone will get their hands on +1 Blackiron Scale Armour. It's my experience that module writers are seldom very good at coming up with riddles, and this one is as turgid as any I've ever seen, but for some reason players seem to enjoy the crappy verse anyway as long as it's not too frequent and relatively easy to solve. It's probably because it's an easy win; solving the puzzle is a quick sop to their ego, and its isolation prevents them from being exposed to easier or harder challenges of a similar ilk by which to scale their success.
As a trope, the riddle doesn't make sense. Once again there's the question of how a magical guardian is easier to come by than a lock and key, and on top of that you have to ask why anyone would restrict access to a valuable item based purely on an intruder's puzzle solving activity. Is being robbed less annoying just because the burglars were amateur cryptographers?
Finally, in amongst all the ridiculous riddles and illusory walls, Keep takes a moment to insert some unwanted realism by reminding the DM that taking an extended rest in most places in the crypts will result in a random encounter; however, this room, hidden behind the illusion, is comparitively safe.
What it doesn't remind you of is that it's a five minute walk back to the keep surface, where it's perfectly safe to rest, and that players who choose to camp here rather than walk thirty metres back up the corridor are the most terminally lazy adventurers ever.