Could you please check these sentences for me, please? Thank you.

1) The story takes place in Prince Prospero’s castle, a big fortress with thick walls and refined furniture.
2) There are seven rooms for parties, each of which has a Gothic window and ornaments of the same colour, except for the last one, whose dominant colour is black but whose window is red.
3) The main character is the Red Death, which appears under the form of a man in a red costume at the end of the story. Another main character is Prince Prospero, a very rich and refined person, who decides to protect himself and his friends from the Red Death in his castle.
4) The other characters in the story are Prospero’s friends, who are always seen not as individuals but as a group.
5) The initial situation is that the Red Death has already killed a lot of people and that Prince Prospero has decided to protect himself and his friends by staying in a strong fortress where nobody could get either in or out.

1. OK

2. Move the comma from after "one" to after "black."

3. Change "under" to "in"

4. OK

5. Needs to be divided into 2 sentences with a good transition word/phrase.
http://grammar.ccc.commnet.edu/grammar/transitions.htm

Sure! Here are the corrections for your sentences:

1) The story takes place in Prince Prospero’s castle, a large fortress with thick walls and elegant furniture.
2) There are seven party rooms, each with a Gothic window and ornaments of the same color, except for the last one, which has a dominant black color but a red window.
3) The main character is the Red Death, who appears in the form of a man wearing a red costume at the end of the story. Another main character is Prince Prospero, a wealthy and sophisticated individual who decides to protect himself and his friends from the Red Death in his castle.
4) The other characters in the story are Prospero’s friends, who are always seen as a group rather than as individuals.
5) The initial situation is that the Red Death has already killed a significant number of people, and Prince Prospero has decided to safeguard himself and his friends by staying in a fortified castle where nobody can enter or leave.