堡 bǎo / pù — an earthwork; castle; position of defense; stronghold; character used in place names
山顶上有一座古老的城堡。Shān dǐng shàng yǒu yí zuò gǔ lǎo de chéng bǎo.There is an ancient castle on the mountaintop.
Common words: 城堡 (chéng bǎo)、堡垒 (bǎo lěi)、地堡 (dì bǎo)