The city-formers were all really fortresses. We did see a transforming castle in Generation 2, although it wasn't a Transformer (but it did do battle with G1 Megatron)... but it was still a wickedly awesome transforming fortress. I guess the closest we've ever had to a Transformer castle would be Boltax - although technically he was a temple - a castle would've been far more fortified, but of course Boltax and his cultists were all raving pacifists. And we all know where that got him!

Anyway, this is the thread about what kinds of alt modes you like, not which alt modes you would want to see (see the other thread for that).