I'm struggling on DeGroot Keep. Are there any specific strategies to win rounds consistently?
There are certain builds that are clearly better on this map than others.
All your drink items can be used as Secondary items. Mad Milk is likely your best choice here, followed by Bonk! Atomic Punch. Crit-a-Cola's downside is too great to recommend using it.
Sandman and Wrap Assassin projectiles still work. Just keep in mind that Sandman has lower health and Wrap Assassin has lower DPS.
The Soldier is one of the weaker classes in Medieval. If you do choose to go Soldier, I recommend the Equalizer or Disciplinary Action. The Disciplinary Action can be used to move Heavies to the front line more quickly than the other team expects.
Consider using the Powerjack (restores health on kill), Back Scratcher (does more damage and heals more from health kits), or Sharpened Volcano Fragment (sets people on fire).
Demoman is easily one of the more powerful classes in Medieval mode.
The Chargin' Targe and Splendid Screen are both good choices, but I would recommend the Splendid Screen, as a swing immediately after hitting someone with the shield is still a crit.
You should use the Ali Baba's Wee Booties or Bootlegger. They have identical effects.
Eyelander/HHHH/Nessie's Nine Iron can be good once you start getting heads. Half-Zoitichi is the best choice unless all the other demos are also using it. Persian Persuader recharges your charge meter twice as fast.
Ullapool Caber is an option, but it makes you a 1-hit wonder.
Note: The Demoman on BLU can jump the castle wall with a charge up one of the hills in front of the castle.
Consider working with a Disciplinary Action Soldier to surprise the enemy.
The Sandvich or Steak Sandvich should be equipped. Most people seem to prefer the latter, but don't forget the standard sandvich can heal half your HP oif you throw it and grab it, with no downtime.
The KGB truly shines in this mode and is by far your best choice.
Don't bother with Engineer.
Crusader's Crossbow is one of two ways to heal people in this mode. Use it. Remember that enemy Spies health meters won't change if you shoot them (it actually hurt them).
The Amputator was created specifically for this game mode. It's one of two ways to heal people. Make sure that your back is to a wall before taunting to heal, or you're an open target for Spies. Preferably, try to remain out of Snipers' sight, too... just remember that Snipers don't have zoom scopes in this mode.
The Solemn Vow is also useful, because you can see enemies health with it and let your teammates know.
Sniper is easily one of the more powerful classes in Medieval mode.
The Huntsman. It's one of two ranged weapons that can be used, and by far the more damaging of the two. Huntsman arrows can be set on fire from the various torches on the map (RED battlements and BLU hill).
Razorback is your main bet, as it blocks one backstab from an enemy Spy. Darwin's Danger Shield is also an option, but it likely isn't going to help you versus a charging Demoman or Spy (your main two opponents to watch for.
The Tribalman's Shiv does bleed damage to these pesky Spies that will try to sneak up behind you.
Spy is easily one of the more powerful classes in Medieval mode.
The Dead Ringer is a surprisingly useful choice on this map. Just make sure you aren't seen/heard when you decloak.
The other cloaking devices are also pretty good, particularly when paired with the YER.
Silent Stabs are very important on this map, moreso than most others because you'll be moving between points.
Your Eternal Reward is your main option on defense. The point have little hidden areas to the sides. Use them to hide and then backstab the other team when they arrive. You can also get behind enemy lines and take out their snipers.
Spy-cicle may also be good, but the giant ice statues you leave behind are an obvious warning to the other team.
The YER and Dead Ringer make a fantastic combo on this map. Here's a replay of me getting 15 backstabs in one round on Degroot Keep
I've always found a heavy equipped with KGBs and the Buffalo Steak Sandvich is a pretty potent combination. It allows you to run in under the effects of the sandvich, kill one person, and then crit the other team to death.
Spies are incredibly successful on defense - half the time it turns into a goofy sniper battle, the other half of the time you've got the one great choke point that everyone will run to, and the third half of the time the game is over in thirteen seconds via wizardry.
Secondarily, for both offense and defense, a good melee Demo loadout is very, very common, and if you're any good with the Hunstman, this is the map to abuse it.
EDIT: You mentioned you're a bad sniper. I'd aim at a good Scout (the Sandman is one of the few ranged weapons you can use), Demo, Heavy, Medic, or Spy. Soldier's not bad with the Equalizer, but I'd avoid Pyro and Engineer like the plague (unless they have some surprising melee weapons I happen to not know exist).