I would think my mental landscape's prominent feature is the castle. For one, it suits my historical tastes, and two, my will to secure and protect my own private thoughts. I also love a mountainous landscape, with the presence of lakes, rivers, deep dark forests and starry nights. Not only are such things beautiful, but a castle fits perfectly into such wild landscapes whilst offering protection from the natural elements. I like nature, but i also enjoy comfort and perhaps a bit of luxury. A castle offers such in an environment of wild and beautiful nature.
I imagine the inner workings of my mind to be like a series of inter-connecting rooms. You could walk into one room and it would be themed around, say, my interest in archery. It would exhibit my bow and arrows, photo's and video's of my successes and failures at the sport, and probably include a shooting range. But there would be other connecting rooms. One door would lead to memories concerned with the people who i have met through archery in the past, another would lead to people i have met in the present at university for instance. Another might lead to an armoury.
Indeed this network of inter-connecting rooms would be vast and of course defy the laws of physics. It would be very much like the TARDIS, but i would know how to navigate my way through these "Halls of Memories." Anything possible that my mind can conjure up would be possible in my castle. I could for instance have a door which leads to the world of the Elder Scrolls, another to the real world, past, present or future, the world of Lord of Rings or Star Trek, or worlds of my own mental creation.
I would probably amass quite a collection of curiosities taken from these worlds. Wabberjack, mithral armour, unobtanium, the Electro-Gun from Killzone 2, vampire dust, silver bullets, enchanted rings and magical potions. I would have those kind of things.
But yeah, anything would be possible in my castle. There would certainly be Iron Maiden concerts in the Great Hall, stand up comedy performances by the likes of Billy Bailey and Dara O'Brien, drinking parties, lectures by Aristotle and public interviews with anyone from Winston Churchill to Dr Who. My friends and family would be there if it would suit them, and we'll have a good time.
That is a picture of what part of my mental landscape could look like, the castle would look something like this. (It's Eilean-Donan castle in Scotland)