I built some quick and easy ruined towers recently.
For the main curved structure I used a cardboard cylinder that whisky comes in. It has a decently sized diameter for a few miniatures to fit inside. I built around the cardboard on either side with expanded polystyrene (packaging foam) and coated it with a PVA, flour and salt mixture. I also put rocks cust from plaster in rock moulds to make the tower feel a part of the landscape more. Finally I added weathering with various Vallejo weather products and also panel lined with a black ink between all the bricks to enhance contrast.
Some more angles. They work well in a Skyrim setting for Elder Scrolls Call to Arms since they are cylindrical towers (would also work well for Middle-Earth SBG even with the slightly smaller scale).
MiniPainting 