One conjecture about why the dragons created Mousetrap Realms was to conceal something else behind them. In this sense, the Mousetrap Realm becomes a form of Interpolation, the magical art of creating a new object between two existing ones. At the dawn of history, the Bridgeworlds, the first human intersphere empire, used the art of Interpolation to build a chain of Fortress Realms as center-realms acros a chain of spheres. (This "chain" of course was many different chains of spherepaths that radiated out from a central hub. It is said that the foundations of the Library of All Worlds are drawn out similarly to one of these Fortress Realms.)
What were the dragons up to when they built the Mousetrap Realms? They may have been concealing their own Nest Realms behind fortress walls. They could interpolate Mousetrap Realms between existing worlds on a spherepath.
For example, suppose a certain World A at a certain point in a chain of worlds; let's call World A, the Spider's Web. The adjacent World B might have been the Nest Realm known as The Red Hatchery of the dragons. Through the art of Interpolation, a Mousetrap Realm - No Floor - might have been inserted between World A and World B. No Floor is conjured into existence as the center-real between the other two, a so-called World A-Prime.
Thus a chain of spheres can be broken. The Mousetrap Realm of No Floor becomes a deadly dead-end for most Spherewalkers going through a gate from there towards No Floor. Meanwhile the dragons cavort or sleep safely in The Red Hatchery beyond. The Red Hatchery no doubt has other gates that go other places. Or it might have not gates. Some dragons can still make their own gates. And they have worked out numerous serpentine paths to get around.