For a first, I think you really showed your understanding of brick count conservation! Typically I see first builds use a lot of 1x1x5s and such instead of the bigger bricks because, y'know, the mindset is more likely "let's just fill it up as quickly as I can" than "how efficient can I be". Like, for example, the interior walls are nice and large, and the exterior uses the brick pattern fairly well. Good work!