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It is access to the outside, Tilly and they need to be kept in for a little while yet. They need to be fixed before they go out and they are still very little. Although we do not use that door much - people sometimes come to it so it is a risk and there are places they can hide. It is also access to the upstairs, which is not quite kitten proofed yet, but is coming on so they will shortly be allowed to roam. For the first month, their room is the snug - thereafter they will have free run downstairs (apart from the kitchen where they may get hurt) and once they are happy with downstairs, they will be able to go upstairs as much as they want. By Christmas they will have the entire house at their disposal.
Plus, they are so little and so silly, I need to be able to supervise them for now.