As described above, the construction is done with blocks. So we construct the level by putting together different type of blocks. The goal of this method is to standardize the levels, and it will be more easier to make levels in the futur.
The drawback is that the levels may look a little cubic, but the goal is not to make the best looking game so it's not really a problem, all the people that have started this kind of project will tell you that it's better to start with low objectives or you won't be able to finish it.