"Others can come in and enjoy it with us, as long as when it comes our time it's there for us," Mrs Collard said.
They bought the 0.4ha section in Turakina Beach Rd years ago, and developed it with a raised building platform, a small lake and plantings.
Building began a year ago, and no finish date has been set. The next additions will be a roof and aluminium windows.
The castle was designed by Mr Collard, with engineering input. Including requirements like chair lifts and disabled toilets and getting consent from Rangitikei District Council has been "quite a battle", he said.
The exterior is made out of aerated concrete imported from Australia. There will be a total of seven bedrooms, a self-contained apartment in each of the two keeps, a grand hall for entertainments and a shared kitchen/lounge area.
They plan to have solar panels and be off the electricity grid, and use the cellphone network for communication.
The Collards have been flooded four times in their Whangaehu Village house. They made sure this one was above the Turakina Valley flood level.
The grounds will have gardens, with ducks on the small lake. The whole will be surrounded by a high fence with automated gates.
"It will be friendly, in the country, safe and secure. There are too many stresses for the elderly now."
Mrs Collard intends to provide the care and activities herself. Mr Collard will carry on the motor and contracting busines, and add a working 1935 Mercedes Benz to the grand hall as decoration.
The two will be calling themselves Count and Contessa Dralloc (Collard spelled backwards and sounding more exotic). They have thrones and a table from a German castle stored away, ready for the medieval-themed opening.