This is a description about the difficulties of maps, and what most of them have.
Basic[]
Basic maps consist of very long tracks, with many loops and circles, and nearly no obsticles blocking you from placing things
Intermediate[]
Intermediate maps consist of decently long tracks, with some loops and circles, but there is usually VERY LARGE amounts of placement restriction. You usually can only place towers in small islands of sorts.
Challenging[]
Challenging maps consist of decently long tracks, with some loops and circles, but there are two entrences and exits, meaning you have to set up two defenses. There also are some obstacles blocking you from placing towers in certain areas.
Expert[]
Expert maps consist of very short tracks, with very little or no loops and circles, and have two entrances and exits, meaning you have to set up two defenses. There also are a decent load of obstacles blocking you from placing towers in certain areas.