noun
Baseball term for a base. Used in compounds like 一塁 (first base), 本塁 (home base), and 塁審 (base umpire).
ランナーが二塁に進んだ。
The runner advanced to second base.
彼は盗塁に成功した。
He succeeded in stealing a base.
noun
Literary or historical term for a fortification. Often appears in compounds like 塁壁 (rampart) or 孤塁 (isolated fortress).
敵の塁を攻め落とした。
They captured the enemy fortress.
ベース is the common loanword for 'base' in baseball, used interchangeably with 塁 in many contexts, but 塁 is more formal and appears in compound terms.
要塞 is the standard modern word for 'fortress'; 塁 is more literary and often refers to an earthen rampart or a specific part of a fortification.
The kanji 塁 originally depicted piled earth and is associated with ramparts and fortifications. The baseball sense is a semantic extension from the idea of a 'base' as a defended position.