Level progression has been outlined to introduce gameplay concepts incrementally then increasing the difficulty of those concepts. The diagram above is an outline of level milestones that can be further expanded in between. You can see one of the basic modifiers of level progression, the number of cells. The number of cells that can be managed increases with every level. Other modifiers are:
Number of Fronts – the number of fronts the building is attacked from.
Building Height – player soon realizes that low buildings are harder to defend.
Scroll Management – player learns juggling between scrolling around sideways to find salvage and telegraph incoming waves.
Expansion – Occupying a neighboring building in order to meet room requirements adds to both scroll management and increasing the fronts to 4.
Broken Cells – During an apocalypse buildings cant be expected to be fully intact. Some cells will be damaged and unusable. They can also be entered from outside without being blockable. These can be placed as we please in level design.
Random Short Spawn Distances – To help mix things up, the players comfort with the telegraphing of enemy spawns from the edge of the level is challenged.
All this comes together in the graph below which shows what will hopefully result in a pattern of increase in difficulty during a level introducing new concepts before dipping slightly in the following level where nothing new is introduced.