Those of you that are familiar with Hirst Arts will be aware that one of their Gothic Molds includes an iron gate, that can used as a portcullis feature in your dungeons.
Bruce Hirst, the owner of Hirst Arts, has this to say about the inclusion of the iron gate on one of his molds: "I placed the iron gate on this mold because I thought it needed one badly. Most crypts, graveyards or cathedrals have gated off areas to keep people out of the holy places or to protect relics."
Portcullises and bricked-up passages are two dungeon elements that get nowhere near as much play as they deserve. In those few instances where portcullises are used in a dungeon, they are typically employed as a trap, with a portcullis dropping behind the party after they open a false door at the end of a long hallway.
Few people think to use a portcullis in lieu of a door. In some ways, I think a portcullis is actually superior to a door. It impedes transit in the same way as a door, but entices characters with a glimpse of what is beyond, while frustrating attempts to search or navigate the area. And because there are harsh rules for bending bars and lifting gates, it is more difficult to bypass a portcullis.
Under the AD&D rules, successive attempts to open a door are permitted. Failure simply increases the danger of wandering monsters, and alerts those beyond the door to your presence. A bend bars/lift gates roll, by comparison, can be made only once: failure precludes further tries. This heightens the tension around the table when a bend bars/lift gates roll must be made. The worst that can happen with a failed open-door attempt is that the monsters beyond the door cannot be surprised. With a failed bend bars/lift gates attempt, you may never gain ingress to that area; an area important enough to have a portcullis guarding it.
When including portcullises in a dungeon, it's important to provide some mechanisms to unlock or otherwise bypass them. Dungeon Module B1, In Search of the Unknown, contains just such a mechanism, found in a storeroom filled with implements. As the DM rhymes off the items in the storeroom, she will eventually announce the existence of four hacksaws. I'll bet your party didn't think to bring those into the dungeon, particularly if they aren't listed in the miscellaneous items for sale.
Other options for bypassing portcullises include levers, found elsewhere in the dungeon, that lock or unlock certain portcullises, depending on their position. The important thing here is to allow the resourceful player to bypass the portcullis, either with a pre-determined dungeon element or through the employment of some clever stratagem (think of Pirates of the Caribbean's Jack Sparrow, using leverage to dislodge the door to his prison cell, for example).
If portcullises suggest something worth protecting, walled-off or bricked-up areas suggest something that characters need to be protected from. Ghouls, vengeful ghosts and other undead immediately come to mind. Bricked-up areas can be even more player-maddening that those defended by portcullises. Clearly, those who built the wall were fearful, embarrassed or ultra-protective of whatever lay beyond. With a portcullis, the builders imagined that they would want to eventually access the blocked area. With a wall? That sends a pretty clear signal that the builders have little intention of ever returning to the area so blocked. Are the characters willing to break out the mattocks and shovels, dig through the wall, and brave the potential horrors beyond? A delicious conundrum, don't you think?