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The listed "status" applies only to existing structures. Your query as to what is classed as "curtilage" is therefore of no real issue, but as only the Tower itself has been classed as the curtilage, if it were an issue, listed building consent would only apply to works carried out to the Tower itself.
However, a new development/build in your garden will generally need planning permission. Listed building consent will only be required if the development is attached to the Tower. The impact on the character, appearance and setting of the listed building will be taken into account when the Council decide whether to give approval for the planning permission.
Depending on what you wish to build, you may require conservation area consent and planning permission.
Please note, Councils are usually pretty good at giving initial advice, so I would advise that you speak to them for your peace of mind.
I hope this asists and answers your question.