I knew what I wanted for E-3 quite early in the process. I knew I basically wanted a closed castle against 50 elite enemies - the only chapter in the game without unpromoted enemies, as it happened - with minimal reinforcements, attacking through two sides until hitting the centre (originally the dead centre, now of course the top). The concept had actually been in my head for several years, in my mate's playthrough of FE4, where he remarked that Julius Plus The Dark Warriors was - actually getting Julia back notwithstanding - actually a very strong final boss encounter, embracing that Julius himself is basically a procession if you get to him with the right person, allowing your star to push into the base while your lesser lights run interference. What could have improved it, he said, was making them respawn. So, that concept of 'relatively easy boss battle but respawning Dark Warriors' was in my head for years, and was always going to be the final battle; I'm pretty sure it was in the original list of 12 chapters, too.