An SD or Medieval contest sounds like the worst idea. Yay, let's pair up the two least fun modes! (actually, that's an unfairly nice thing to say about CTF. I'm sorry SD, you're not as bad as CTF.)
Sometimes mixing the 2 most awful modes can result in something awful because they are able to counter the problem that each mode gives:
Medieval: there barely is a target and some classes clearly are stronger. Sniper, demo, heavy and spy are somewhat the only usefull classes due to their abilities to kill fast. The soldier provides support with the banner and the medic for healing. This still makes the scout, pyro and engineer obsolete.
SD : speed and the ability to hold a point becomes important (scout, pyro can be added due to mobility, medic can be added for having a more effective healing moment), and engineer can still be usefull as he just like other classes is fast enough. He however still lacks a decent medieval weapon.
So mixing 2 modes can improve a mode to be better.
For medieval the main issue is that it lacks the use of all classes and only focusses on the killing part (the objective is more like decoration rather than an objective), thats the first thing to counter on that mode to make it fun. Without that i would call your map doomed as it will play exactly like degrootkeep and basicly remove the need of the special objective.
For SD minisentries are hated to death (since the map is short in enemy to enemy paths they are often rebuilt fast). Medieval fixes that by simply not allowing those anoying features (and with proper cover the huntsman cant do alot either).