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. Since its rebalancing in 2013, Crit-a-Cola may be worthwhile.
Sandman and Wrap Assassin projectiles still work. Just keep in mind that Sandman has lower health and Wrap Assassin has lower DPS.
Boston Basher/Three Rune Blade works well if you can consistently hit enemies.
Only the Soldier's banners can be equipped. Your best bet is likely the Battalion's Backup, although the Concheror could also be useful.
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 or other Soldiers to the front line more quickly than the other team expects.
The Equalizer's damage goes up as your health goes down. However, a Medic cannot heal you when you're using it.
You may be tempted to use the Half-Zoitichi, but if you're going to do that, I recommend playing DemoMan instead.
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.
You should use the Ali Baba's Wee Booties or Bootlegger. They have identical effects.
The Base Jumper can be used, but it's not really that useful.
The Tide Turner is your best choice. Whenever you kill someone with the crit during or after the charge, you regain 75% of your charge meter.
The Chargin' Targe and Splendid Screen are both good choices as well.
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 as it heals you to full with every kill.
Persian Persuader recharges your charge meter twice as fast.
With the Tide Turner, the Claidheamh Mòr is also an option. It increases your charge distance slightly at the expense of 15 health.
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 be eaten to heal your health back to full or thrown to a teammate to heal them. Since enemies drop health kits on death, you can get another Sandvich fairly easily.
The KGB truly shines in this mode and is by far your best choice. You get 5 seconds of crits for every kill with it. The Warrior's Spirit is good to team up with the Buffalo Steak Sandvich. The Holiday Punch may give you an unforeseen advantage, since all criticals stun your foes and allow them to easily be disposed of. It also always crits from behind. Pair it with Steak Sandvich for it speed boost (you move slightly slower than Medics while it is active).
Don't bother with Engineer.
Crusader's Crossbow and its holiday version are the only primary weapons that a Medic can use in Medieval mode. It 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)... and if you have damage meters turned on, hitting a Spy will not make the green + health appear over their head.
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 and its reskins. It's the only primary weapon a Sniper can use in this mode. 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 Cozy Camper can also be used, but it really isn't worth it.
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).
to be honest, Ive never played that game mode, but i do kinda know what it is about.
soilder use the man treads and equalizer. they can't stop your advance and you mine their heads off. disciplinary actions damage reduction is not that much too, and makes you a great tema player.
medic crossbow and amputtator all the way. nothing is better. try to get the set bonus too
engineer if you are stud enough to use engineer, go with the gunslinger. +25 health and crits every 3 hits? definitely. but, again, I've never played it, so i don't know if it works on midevil