When you move into the new dungeon, you need to choose one of three sets of traps which you can buy for gold, orbs or emeralds. With the two first sets, you also get a pre-made traps placement.
The "Random traps" set gives you 3 random traps, and you can change any of them by spending orbs if you don't like the ones you got. Note that you cannot have 3 of the same traps in the random set, and you have to create your own traps placement for the "Random traps" set. When you get them, you have to build your own defence because there is no standard (automatic) placement in this set.
You can change the set of traps in Edit mode.