Ответ:

2. Was there a garden outside the castle?
Yes, there was.
3. Were there two flags on top of the castle?
No, there were three flags
4. Were there two fountains outside.
No, there was only one
5. Were there any horses there?
Yes, one near fountain

Объяснение:

множественное число и единственное число
2. "Garden" - единственное - was
3.  Flags (flag + s) - множественное - were
4. fountains (fountain + s) - множественное  - were
5. horses (horse + s) - множественное - were