The cramped corridors and all the zombies pouring in was a great touch, as was the persistent threat of the gargoyles, but I'd have moved the skeletons from the ground to the top floor to make the final battle a bit more intense.

If you're worried a wight and two skeletons are too much for a third level party after all those zombies, you could add a short side quest that suggests the gargoyles are enchanted sentries for the tower and there's a command word (found in the captain's quarters perhaps) that makes the gargoyles recognise you as friend not foe and they could help against the enemies in the tower. You might want to give the gargoyles reduced hit points if you do that, though, to reflect that they've been fighting the zombies and skeletons long before you arrived.

For the sake of tension and atmosphere, the ground floor doesn't really need enemies as much as it needs visual clues. Perhaps the entrance to the tower could have been torn open and there could be evidence of attempts to repel the attacking force, such as barricades, fallen skeletons and dead guards. Make it a real horror movie moment, showing these people fought and died but took a lot of enemies with them.

I assume the intention was that the wight had killed everyone in the tower and raised them as zombies under its control, so evidence of that would really add a lot to the tower as a whole. Perhaps all of the zombies could be described as wearing the same armour, indicating they were the soldiers stationed here and they all fell in the attack.