fort, rampart, walls, base(ball)
壘 centers on a defensive structure built up from earth: a fort, rampart, or wall. The baseball base sense is a modern extension of that same idea of a raised earthen marker.
壘 combines 土 (earth) with 畾, a component that likely represents piled-up field-like structures, to express an earthen rampart or fortification. The modern simplified form 塁 replaces the upper part with a simpler shape.
Three fields 畾 stacked on top of earth 土 suggest layers of soil piled high to form a protective wall or fort.
For ルイ, imagine a fort built from layers of earth that look like a giant layered cake: the cake is a 'rui' (ルイ) cake, and the fort's walls are just as sturdy.