A medieval fortress towers on the hill.
その城塞は難攻不落で知られていた。
The stronghold was known to be impregnable.
城 is the general word for a castle, which may or may not be heavily fortified. 城塞 emphasizes the defensive, fortress-like aspect.
Standard kanji spelling for fortress or stronghold.
Alternate kanji spelling; 砦 (fort) emphasizes the defensive structure.
Compound of 城 (castle) and 塞 (fortress, stronghold). The kanji 砦 in the alternate spelling also means fort or stronghold.