Castles are fine defensive structures, that allow a relatively small number of defenders to hold off a much larger attacking force. The defenders need to be:
- sufficiently numerous that they can maintain constant watch on all approaches;
- armed with missile weapons and (at close range) heavy and/or heated objects that can be dropped on attackers; and
- provisioned sufficiently that a besieging force will get tired of sitting outside the gates before the garrison runs out of water, food and other essential supplies.
Castles are less effective as defensive structures once heavy anti-materiel weapons are available or air mobile forces are able to bypass the battlement defences.
A single griffin can exploit surprise and aggression to attack a castle and eliminate a disliked lord and some of its defenders by crashing through the roof of the hall and running amok. However, if the griffin is so large that there are doors that it cannot pass through (as specified by the OP) then any quick, intelligent occupants of the castle will flee through such doors and hide in areas the griffin cannot access. Such occupants will probably not remain in the castle indefinitely, but will flee at the first opportunity while the griffin is sleeping, hunting, or watching a different small-sized doorway. When they leave, they have no reason to close any door / portcullis etc behind them.
Sooner or later someone decides to take the castle away from its griffin lord. This someone talks to survivors, sends in some scouts to make observations and then attacks in force - probably archers protected by pikemen. The griffin is now in serious trouble:
- The griffin cannot maintain a watch on all approaches - it can only watch one direction at once and it needs to sleep.
- Even if it could watch in all directions (or gets lucky and spots the attack force approaching) the castle is of no use to it - it relies on teeth and claws and has no ranged weapons to employ against the attackers.
If the situation did somehow develop into a siege, as soon as the griffin was seen flying out to hunt or get water the attackers would move in and occupy the castle. The griffin's mobility allows it to flee a losing battle, but as the sole defender it cannot afford to resupply.
The inevitable conclusion is that the attackers will seize the castle, either over the griffin's dead body or when the griffin flees. What condition will the castle be in? A thatch roof will last one or two years maximum before it rots or blows away, some wooden shingles will last longer but will degrade over time without maintenance. Once the floors are unprotected from the rain and rot then all upper level floors will degrade, eventually leaving only the ground level stone floors intact. In short, if the attacking force takes more than a few years to arrive then they will probably be unopposed - the griffin will have left in favour of a snug cave rather than remain in a rotting castle.
The only ray of hope for the griffin to hold the castle is if it is a griffin-mage, as per various fantasy series. A powerful griffin mage can conjure wards to alert it to approaching enemy, set magical booby traps on some approaches and cast overwhelming, ranged combat magic at especially important enemies from the castle battlements. But... if the answer to all the problems is "really, really powerful magic" then the mage could be a human, elf, orc or awakened parrot - the question is no longer about a griffin.