城堡(chéng bǎo) - castle, fortress in Mandarin

城堡 (chéng bǎo) is the European style castle: turrets, moat, drawbridge. It is the word used in fairy tales, in travel writing about Europe, and in games, and its measure word is 座, the one for large fixed structures.

Chinese fortifications get different words. A walled city is 城, the Great Wall is 长城, and an imperial palace complex is 宫 or 故宫. Calling the Forbidden City a 城堡 would sound like a translation error.

Note that 堡 has more than one reading. As bǎo it means fort, as in 城堡 and 碉堡, but in place names it is often read pù, and in the loanword 汉堡 (hamburger, also Hamburg) it stays bǎo. If you meet the character alone, check the word before you guess the tone.