Only the front door. Not for security reasons but because our dog can open it and does, so to avoid him getting on the road we keep it locked. As I write three patio doors are wide open and the kitchen door, also windows in all bedrooms and bathroom. DH is far more security conscious than me. He locks the doors at night and shuts windows. We can't lock any windows, never had keys for them. But we often forget to shut doors or windows when we go out, and have left the car unlocked countless times. We have an alarm system (it was an insurance requirement) but we've never used it, can't remember how it works now. When we moved here the side of the house was open to the road, so neighbours and postman etc always walked round to the back door, a rural custom I was told, I quite liked it. DH didn't like it so he installed a side gate. I couldn't live in a house with all the doors and windows closed and locked all the time, I'd feel like I was in a prison!