Draw the outline of the castle on white paper. This outline must resemble an architectural design, as you have to mention each element's dimensions. Present the outline to the class and ask for suggestions.
Divide the class into as many teams as the castle's parts. For example, a team of four can be responsible for the watchtowers, another team of four for the walls and a third team for the structures in the castle's interior. Write down each team's members and its specific task in the project.
Ask the members of each team to divide the tasks between them. For example, one student can draw shapes on cardboard, another can cut them out, a third one can color them and another student can glue the parts on the castle. Allow teams to make their own decisions.
Provide each team with cardboard for the task. As you are already aware of what task each team has to undertake, you know how much cardboard they need. However, accidents or miscalculations can happen and students may need additional cardboard, so keep a few extra sheets just in case.
Warn students when you see that a part they have created does not match the original design. A single wrong part can give an awkward look to the final result.
Set the order in which each team can add its part on the castle. The first team must be the one responsible for the walls, followed by the entrance team, then the watchtowers team and so forth.