  We clarify the conditions for the emergence of multiple-${\bf Q}$ structures
out of lattice and easy-axis spin anisotropy in frustrated magnets. By
considering magnets whose exchange interaction has multiple global minima in
momentum space, we find that both types of anisotropy stabilize triple-${\bf
Q}$ orderings. Moderate anisotropy leads to a magnetic field-induced skyrmion
crystal, which evolves into a bubble crystal for increasing spatial or spin
anisotropy. The bubble crystal exhibits a quasi-continuous (devil's staircase)
temperature dependent ordering wave-vector, characteristic of the competition
between frustrated exchange and strong easy-axis anisotropy.
