Is the word castelo the same in Brazilian and European Portuguese?

Yes

The word "castelo" is identical in spelling, meaning, and grammatical function in both Brazilian and Continental Portuguese. The only difference lies in pronunciation. In Brazilian Portuguese, the vowels are typically more open and clearly articulated. In Continental Portuguese, unstressed vowels (such as the "e" in the middle and the final "o") are much more reduced, shortened, or even nearly silent, which gives the word a more "clipped" or "compressed" sound.

Brazilian Portuguese Examples

1. Eu visitei um castelo muito bonito. (I visited a very beautiful castle.)
2. As crianças estão fazendo castelos de areia na praia. (The children are making sandcastles on the beach.)
3. O rei mora em um castelo enorme. (The king lives in a huge castle.)
4. Você gosta de visitar castelos antigos? (Do you like to visit old castles?)
5. Ele está construindo um castelo de brinquedo. (He is building a toy castle.)

Continental Portuguese Examples

1. Eu visitei um castelo magnífico. (I visited a magnificent castle.)
2. As crianças estão a fazer castelos de areia na praia. (The children are making sandcastles on the beach.)
3. O rei habita num castelo enorme. (The king inhabits a huge castle.)
4. Tu gostas de visitar castelos antigos? (Do you like to visit old castles?)
5. Ele está a construir um castelo de brinquedo. (He is building a toy castle.)