Galaxy formation in the first billion years mark a time of great upheaval in the history of the Universe: as the first sources of light, these galaxies ended the ‘cosmic dark ages’ and produced the first photons that could break apart the hydrogen atoms suffusing all of space starting the process of cosmic reionization. As the earliest building blocks, the galaxies that formed in the first billion years also determine the physical properties of all subsequent galaxy populations. However fully coupling galaxy formation with large-scale reionization remains a massive computational challenge as a result of the range in (mass and volume) scales required. I will start by introducing the Astraeus framework that is the largest (230 Mpc box) and most highly resolved (DM resolution mass of 10^6.9 solar masses) simulation to fully couple a semi-analytic model of galaxy formation with a semi-numerical scheme for reionization (feedback) to shed light on galaxy formation at cosmic dawn. I will then show how cross-correlations of 21cm data with the underlying galaxy population, in the forthcoming era of 21cm cosmology, will yield tantalizing constraints on the average intergalactic medium ionization state as well as the reionization topology (outside-in versus inside-out). Finally, I will try to give a flavour of how the assembly of early galaxies, accessible with the forthcoming James Webb Space Telescope, can provide a powerful testbed for Dark Matter models beyond “Cold Dark Matter”.