Corridors are available from the very beginning of the game and cost $100 per tile. Like every other room type, they have to be at least 2 tiles wide, which makes sense since doors are also 2 tiles wide. Corridors, despite being the most basic roomtype, have 3 purposes:
1) creating an entrance to the base
The entrance of a base often consists of a labyrinth of corridors and rooms with traps to keep unwanted visitors out of the main base. It is highly recommended to do so. Afterall, who would want a swarm of enemy veteran soldiers to just walk in the heart of the base and shoot up everything, or worse, a super agent? Even if you would defeat them, new agents would be showing up sooner than you can train the military minions you lost in the previous battle. With all those bodybags piling up, your heat would go through the roof and it's only a matter of time before agents overrun your base and eventually take your Evil Genius out: game over. So yes, it is better to keep busy, mislead or trap the overwhelming majority of the agents rather than to kill them all, particularly on hard difficulty.
So, how to trick them? One way is to make several base entrances in the mountain flank, but have only one of them which leads to the actual base. With 3 access points to the mountain, and agents landing on random locations on the island, you'll already have about 2/3 of the agents going into the wrong entrances and make them waste time there.
Secondly, agents absolutely love to know what's behind locked doors, so why not make a bunch of twists and turns in your entrance corridor with an occasional door left and right, which leads to nowhere or into a trap? The main base itself is of course at the end of the entrance corridor, but the agents must have made a whole series of right decisions before they even make it there. Then, instead of killing him, have a social minion "charm" him out of the frontdoor. His time on the island has almost ran its course anyway and he must be bored to death walking around in empty corridors the whole time.
- providing connections between rooms within the main base to better organize the layout, manage the base and improve security
- outdoors in Topside Shacks, to provide fire extinguishers to nearby hotels or insert traps to keep the enemy occupied
While you could connect different rooms directly without the use of a corridor, it is recommended to use them anyway. Not only does it nicely divide your base and keeps things orderly but, more importantly, it highly increases the security within the base.
