A wonderland (also known as a Memory Palace) is an imagined location created with extreme focus on detail, such that the place is perceived as having the verisimilitude of reality, used as a meditative aid, mnemonic device, or as a platform for the interaction or development of tulpas and/or other mental constructs.
Method of Loci[]
The fundamental feature of a wonderland is that it is a self-persistent mnemonic device; that is, when returning to the wonderland, things are as they were before, without any conscious thought required to recall features. The reason the process is known as the Method of loci, is because you're using locations, tying together navigational memory with other sensory information, so that the memory of this perceived location leaves a very strong imprint in the mind.
The creation process of a wonderland is extremely similar to the creation process of other mental constructs, in that details are specified to the point of extreme familiarity, and repeated until those features become a self-consistent persistent process within the mind. There are also many differences between creating a wonderland and creating tulpas and other types of mental constructs.