Hyrule Castle is a location of central importance in all the Hyrule Castles in Legend of Zelda, Ocarina of Time's might be the most poorly designed and manned.
As far as being a defensible position, the castle itself isn't terrible. It's quite small, especially in comparison to the Hyrule Castles of Twilight Princess and Breath of the Wild, but the entire outer wall appears to have a parapet for archers. It also only has one gate and a moat surrounding it. The biggest issue is that it was built in the wrong place. Building a castle in a valley can conceivably have its advantages, but definitely not a valley this small. Building Hyrule Castle on any of the surrounding hills would have been a better option than where it is now. Those same hills can be used to easily breach the outer wall.
The gameplay itself highlights another glaring issue with Hyrule Castle's design flaws in Ocarina of Time. Link, raised as a Kokiri in Ocarina with no conceivable understanding of how to infiltrate a castle, was able to gain entrance to the inner courtyards by crawling through what is essentially a drain pipe. Even Helm's Deep's culvert in The Lord of the Rings had a grate on it. All it took was a single, well-placed crate to allow a literal child across the moat and into the castle.
Hyrule Castle Guards Aren't Good At Adapting
Even more baffling is that Link's penetration of the castle walls is the second invasion of the palace in recent memory, not to mention the castle falls under the power of Ganon halfway through Ocarina of Time. Hylians in Castle Town discuss how security at the castle has tightened after a man tried to sneak in and see Princess Zelda. Link can actually talk to the culprit, a juggler in a blue shirt, who reveals he got past the guards, but became stuck in a drain on the east side of the castle (the Hylian army better hope invaders don't have any soldiers of small stature). Even with the guards on high alert, Link is able to sneak past more than a dozen.
In addition to its design faults, the guards of Hyrule Castle appear negligent. Talon is bringing a delivery of milk from Lon Lon Ranch and falls asleep next to a side door. The guards must not keep a record of who comes and goes, because they fail to kick Talon out until Link arrives and wakes him up with a freshly hatched Cuckoo, Zelda's chickens with violent inclinations. Additionally, once Link gets through the courtyards, there isn't a single guard posted within sight of Princess Zelda - the young girl who was just recently the target of another intruder. Even without the Triforce of Power, Ganondorf could have taken over Hyrule Castle in Ocarina of Time. Gerudo's Fortress is better protected than the Hylian royal family.
