Just a few ponderings,
What is it you want your back garden to be protected from, and do you feel unsafe yourself if you feel the need for protection?
This is a whole subject in itself perhaps others here can advise you better than I, and of course I don't know you, but I would say be careful about setting wards etc and before you do anything intrusive, I think you have to begin looking at the deeper picture first.
When you say the place feels empty that there is no spirit or heart to it, can you describe in more detail why you feel this way? Does it feel more empty at different times of the day or in different seasons.
If it is a nature reserve, who is responsible for it, try to find out why it was made a nature reserve, what is special about it, there must have been a reason it was made a reserve in the first place.
Perhaps try and make a plan of it see what is growing on it and what inhabits it, get to know it in detail throughout the changing seasons. I wouldn't rush into the use of 'Druid Magick' without having a greater understanding of your place, this of course takes time and commitment from you. If you build up a positive personal rapport with it on the physical level, you may find you can begin to communicate on other levels and find out why it seems empty and what is needed to bring the heart back into it. Perhaps the spirits of the place are hiding, perhaps they need welcoming back when the time is right, try meditating frequently there if you can, record your experiences and any thoughts or impressions that come to you - don't go off your first thoughts but do see if a pattern emerges over time. The only thing I would try to do on a more spiritual level is send out your love to the place frequently and see what happens.
Lots of druid stuff is very slow and painstaking, but results will come as you begin to truly know this space.
Let us know what happens!