Stability begins with the base. When you've built the first two metres of the rolling tower, check that it is horizontal. Measure this in terms of length and width. If it is not horizontal, adjust using the wheels. They can be adjusted up and down to create stability.
Once you've built the structure, check that the scaffold is still horizontal. The weight of the scaffold can lead to it sinking if the ground is too soft. When building the scaffold: measuring is knowing (and in this case, safe working).