ATHENA is currently in Phase A, with a view to adoption in 2021. This paper will present the design status for the spacecraft (SC), covering the overall configuration of the SC, as well as the main design features of the main modules. Then the focus will be on the functional and environmental requirements, the thermo-mechanical design and the Assembly, Integration and Test considerations related to the very large Mirror Assembly Module housing the Silicon Pore Optic (SPO) Mirror Modules. Initially functional requirements on the optics accommodation are presented, with the Effective Area and Half Energy Width (HEW) requirements leading to a preliminary HEW budget allocated across the main contributors. This is then used as a reference to derive subsequent requirements and engineering considerations, including: The procedures and technologies under consideration for AIT to achieve the required alignment; stiffness requirements and handling scheme required to constrain deformation under gravity during x-ray testing; temperature control to constrain thermo-elastic deformation during flight; and the capability to focus using the Instrument Switching Mechanism. Next, our best understanding of the launch mechanical environment imposed by the forthcoming Ariane-64 launch vehicle is presented along with the mechanical requirements of the MMs, and the need to minimize shock-loading of the MMs is stressed. Methods to achieve this are presented, including: Modal-tuning of the MAM to act as a low-pass filter during launch shock events; low-shock release-mechanisms; the possibility to deploy a passive vibration solution in the launcher interface plane to reduce loads.