groundwork, foundation, fundament, basis, base, underpinning, building block, footstall, bench table, socle, more...

capstone, apex, peak, summit

foundation, keystone, more...

chief, very, important, second, first, major, precious, fundamental, essential, third, basic

important piece,   foundational work,   founding stone,   foundational stone,   keystone value,   first stone,   foundational piece,   foundation or base,   corner key,   pillar of something