How to restore old permanent sandcastle?

My in-laws have a garden that was inherited by two pervious owners. The garden hosts several intricate sand sculptures. The sculptures range from houses, churches, a lighthouse, and a couple cottages. They also differ in height. I have never seen anything like them before. They look to be made of sand, small smooth pebbles/rocks, and whatever that is holding them together. They once housed lights and some still have stained glass for the windows. I can't tell you how beautiful they are. My father inlaw tried to fix them with maybe concrete and it looks awful. They purchased their house 39 years ago from an elderly couple that only lived there 10 years. The sculptures were there when the elderly couple purchased. Someone said that they might have been built during the depression. Can anyone tell me what they are made of or are these sculptures just someones beautiful idea come to life? Please help?