In The Sims Medieval, you can enable Window Mode to play in, if you would rather not play in fullscreen mode.
Hit Alt+Enter on the keyboard to switch back and forth from window to full screen.
There is also a cheat code to enable Window Mode. Hit Ctrl+Shift+C on they keyboard to bring up the console at the top of the screen. Type in fullscreen off and hit enter.
To go back to fullscreen mode, open up the console again and type in fullscreen on and hit enter.
