In This Question we discussed how likely bats are to become sapient. Lets assume that they could. They would weigh at most 150 lbs and would look much like flying foxes. Assuming they could evolve sapience, how different would their medieval architecture look like.
Architecture doesn't solely depend on intellect and sapience.
The structure of manus (hand) plays the most important role in constructing something once the plans have been laid out. On the paper and in my mind, I could draw a hundred amazing architectural designs which wouldn't be possible for medieval people to build simply because they did not have the technology. How much complexity of design can your bats' hands handle? How much can their wrists turn? The shoulders? The length of their arms? How much are they hindered due to the attachment of skin membrane to their arms?
Another extremely relevant thing is the available raw materials your bats have in their surroundings. Do they have any glue? What is the shape and size of the water stones in their region? Can they manipulate wood with their hands (see above point)? Ropes? If they don't have glue available, how else are they going to keep things together? I hope you aren't expecting them to use nails and hammers ... What is their environment like? Are they living in jungles or caves?
What are their requirements for the buildings? Are they going to build residences hanging upside down from ceilings or upright on the floor? The resulting colony would heavily depend on what they want to make after they have decided what they can make.