Sometimes it is unavoidable to have extra hallways in a dungeon. Whether they are connecting spaces to allow a room to grow in the future, or a spur toward a resource node to mine, you want a way to use those corridors in some manner that benefits your dungeon overlording schemes.
The only real use of a pure hallway tile is to place Traps on it. A maze of traps leading from an entrance can be very effective in disrupting any attacks that come in. If you place rooms that commonly have your range-strikers off one or both sides of your maze of traps, then you start to have a very effective Dungeon Defense going.
Mine tiles Edit
For hallways within 3 squares of an active mining node, a good option is to turn them into Mine tiles. Just by being a mine tile they increase your mining rate by 1%, and if they are up to 3 squares away from a resource node (diagonally too), they can have a stack of that resource build up on them.Remember that if you put a mine tile close to the entrance you leave your entire mine prone to looting by an attacking player or npc.
But for those hallways that really have no direct purpose and are not within 3 squares of a resource node, you should consider making Library tiles. Every Library tile allows one quarter more Warlock, effectively making place for a warlock for each 4 tiles you put down. Adding more tiles over the amount of your Warlocks will increase the research rate if you choose to shock your Warlocks and it will increase the total room available for research to be deposited, which is just as important. You need to have enough space for all the research to be placed between log ins.
Library tiles, as any other tiles for that matter, do not need any proximity to your main Library room, that you have been leveling up, in order to give their bonus. They will grow stacks of research no matter where in your dungeon they are located, as long as there is no furniture in that tile. They are in fact a convenient place to put Library furniture on so that you keep the research stack spawning locations confined in a convenient location that doesn't require you to scroll around. The drawback to this is that a pillage attack on your Library could destroy the furniture if it's placed at a location where attackers can get to it easily, as compared to the main library which likely will be past the trap maze.
Also, consider making any unused hall tiles into farm tiles. You will be able to create a large reserve of food sitting on those farms. The same would be true of currently unused tiles - place them in the dungeon as farms. They are cheap to pick up or convert later. Having a large supply of extra food around is very useful, particularly if you invest in transmuting it into Iron or Crystal when needed. Food will also be one of the very needed Resources later in the game.
Another temporary use of these hallway tiles is to boost your storage capacity for times where you need to upgrade a room that requires a large amount of items that you may not have room for normally. You can temporarily put down Storage Room or Elements Confine tiles to boost your capacity while you ship in (or pick up from a packed mine) the Resources you need. Once stored you can then pick the tiles back up without losing the stored items. For newer players needing 6000 Primordial Earth to upgrade their mine to level 5 this is a good way to gain the space needed that they might not normally have in that dungeon. But you will want to pick up these tiles as soon as you can to minimize your risk of being raided easily.
One thing to remember with this strategy is that when your dungeon is attacked creatures have a chance to spawn in certain rooms, so having Library tiles or Farm tiles outside your trap maze (connecting your entrances along the edge of the dungeon for example) could cause your troops to spawn in areas you may not want them to spawn. Orcs spawn in the Farm quite often, so having your orcs spawn out away from the Den/trap maze could potentially keep one or more out of the defense fight, or worse... on their own as the attacking force enters and kills them.