What Mods are Out There
Now for the fun part, picking out some mods! You'll find that some of the larger sites for Sims 3 mods, such as Mod The Sims, are also starting to get into Sims Medieval mods, you may just have to do a bit of poking around their forums to find them. Here is a good place to get started.
Keep in mind, some of your old Sims 3 mods may work as well! While most will not be compatible, some players have reported things like patterns and default replacement eye mods working without any alterations. If you're curious if your old favorite might work, it might be worth a try! A few mods that work are these eyes, these patterns, and even these extra create-a-sim sliders!
If you're more interested in ways to change gameplay, check out some of the mods at The Simmers Club Exchange. There's tweaks for all sorts of things, from giving you more time for quests to making your sims decide to bathe more. Those interested in more stuff to play with might be interested in reading up on the achievement unlocking mod, since several of the achievements provide you with extra clothes and objects.
Your heroes caught the dreaded blue eye bug? Some mods even provide work arounds for common problems in the game, and provide you with extra power in their customization. Keep in mind however that these powerful mods often come with a higher risk of causing problems with your save game. Be sure to note what files correspond with which mod in case you need to uninstall, and always make backups of your favorite save games!