A watchtower is one of the key features of any fantasy battlefield. You can make one easily with a piece of paper with a photographic texture
This was my first terrain project, all the way back in the 90's. The decade that brought us "The Pretender" and Natalie Imbruglia's "Torn". The time when we believed in the "happy end of history" and our biggest concern was the way cigars are stored in the White House :-)
1. So I took a piece of carboard with a brick-like texture I got from a hobby train store and rolled it up into a cylinder.
2. I made doors, windows and other ornaments from normal cardboard and then painted them to look like stones
3. I used a wooden disc (cut to size) for the to floor. You can get a special drill extension for cutting discs out of planks
4. To finish things up I used some hobby train foliage and actual stones to cover seams and mistakes
This castles were my inspiration: