I know the castle isn't finished yet and they've probably thought about a lot of these features, but here's what I would do to add to the defense of the castle.
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ROAD BLOCKS - Prepare boulders that can roll onto the road leading to the castle. Don't put them close to the castle. Do this far away from the castle. Or you can use other items or just blow some large craters that make them impassible. As soon as everybody is accounted for, block the road.
SILENT INFRARED PERIMETER ALARM SYSTEM - This gives you a heads up so there's no surprise. Once the alarm goes off, call everybody inside the castle into battle positions. Usually, they won't know they've tripped an alarm.
NO TRESSPASSING SIGNS - If it were my castle, I would place these around the perimeter of the castle at the point at which the castle becomes visible. I would write something like, "NO TRESSPASSING. TRESSPASSORS AND THIEVES WILL BE SHOT. TO REQUEST A FRIENDLY ALLIANCE, STEP INTO THE OPEN UNARMED, WAVING A WHITE FLAG."
FALSE COVERS - Fill sandbags with sand and shrapnel. Place them in a location where the attackers will take cover. However, on the castle side, at the base of these sandbags, place explosives that will detonate when shot, like Tannerite (or remote detonated). You want your attackers to take cover in these places. Once they are there, boom.
UNDERGROUND ESCAPE TUNNEL - In a worst case scenario, you can escape through this tunnel. Make sure the exit from the tunnel is not visible and place locked gates or some kind of obstruction to prevent attacks from the tunnel.
TRENCH WITH SPIKES - It's a cheap and effective barrier. About 10 ft. wide is good.
BARB WIRE - Another great barrier. If it is placed right next to the trench, the barriers will multiply their effectiveness.
HINGED GROUND BOARDS - This is a good idea for wherever there is a gap in your barriers -- choke points. As you retreat into your castle, you can swing these things down and they will have spikes on them to fill in the barrier gap. You can also add explosives to the front of them, like claymores or more Tannerite/shrapnel sandbags.
MOAT/PONDS - Used for thousands of years for a reason. The only reason I don't have one moat that goes all the way around is to keep the gene pool of the fish separate in order to prevent diseases from spreading or to allow for different species. I would also put the moat a little further from the castle in order that if people get next to the walls, you can drop oil and fire on the attackers without worrying about polluting your fish ponds. Or avoid these complexities by making it a true moat and put your fish ponds elsewhere.
MURAL/BLINDS - Your defense is great, but with the big castle at the top of the hill, it means you are also visible. How far can you see from the top of the castle? That's how far people can see you. I'm in the mindset that it's better to avoid conflicts than risk death, even if it is a small risk. People will get curious and desperate and some will steal and attack. At some point post collapse, having a huge fort at the top of the hill is ideal, but for the first 5-20 years, it can attract problems. I suggest camouflaging the castle and adding treetop blinds to help eliminate your profile that stands in contrast to the rest of the forest.
SWIVEL SLINGSHOTS - Having these large swivel slingshots bolted in place will extend your range for launching molotov cocktails and bombs.
BUCKETS OF FLAMMABLE LIQUIDS - For as long as people have been sieging castles, the castle defenders have been pouring tar on them and lighting them on fire. It's one of your best up-close defenses. Use molotov cocktails or flares to set them on fire.
MOLOTOV COCKTAILS - These can be thrown or launched at enemies approaching or used to hit people at the base of the castle. Just be careful up there and don't mess up.
GUNS - Obvious. But store them inside or keep them waterproofed on the roof. If you can get some machine guns mounted up there, that would be great. I would easily replace slingshots with turrets anyday.
PIPE BOMBS - If you use more than just guns, your attackers will get more scared. The combination of gunfire, false covers, molotov cocktails, and pipe bombs will cause much more fear.