1
The castle is fantastic and it’s in a very good condition.
The castle is fantastic and it’s in very good condition.
We use in good/excellent/bad/terrible condition, without a/an, to describe the state of something.
2
The car is in very good conditions, but I still think it’s expensive.
The car is in very good condition, but I still think it’s expensive.
Use singular condition when describing the state of a particular object or building.
Use plural conditions for the situation in which people live or work: ‘The employees are working in terrible conditions.’