fort, stronghold, entrenchments
砦 centers on a defensive military position: a fort, stronghold, or entrenched camp built to resist attack.
砦 combines 此 (originally a footprint or here) with 石 (stone), suggesting a stone-built defensive position. The exact historical development is uncertain.
The top 此 looks like a footprint standing firm, and the bottom 石 is stone. Picture a stone fort where defenders plant their feet and hold the line.
For サイ, imagine a fort under siege, and the defenders sigh with relief when reinforcements arrive: sigh -> サイ, as they hold the stone stronghold.
fort; fortress; stronghold; fortification
Siege of Fort Ticonderoga (1777)
to capture a fort
abatis