I can't emphasize enough how easy and accessible Sketchup is. 7th-graders made these castles on their second day of using the program.
Day 1: Students practice with orbiting (O), camera moving (H), and zooming (Z). Then they make a simple house with rectangle tool (R), push-pull (P), and line tool (L). Stronger students try building stairs and bevelling edges with an arc (A). Students produce a complex house exterior.
Day 2: To build a castle, students must learn the offset tool (F) and circle tool (C) for the turrets. They learn scaling (S) and moving (M). This is different from the house because they're building an interior, with rooms and doorways between them.
Day 3: They build furniture: A chair and a desk. Then they build a fancy lamp using the Follow Me tool as a lathe.
Day 4: Now they know Sketchup well enough to direct themselves and to build their own structures. Here's an emergency shelter that one of my 7th-graders built after learning Sketchup (I cropped out the annotations but you can still see the callout lines):
