I was slightly shocked to read the 4e version of a portable hole:
Power (At-Will): Standard Action. Place a portable hole on a wall, a floor, or a ceiling. (The surface must be flat for the item to function.) The portable hole instantly creates a 5-foot-wide, 5-foot-deep hole in that surface. With a standard action, any creature adjacent to a portable hole can pick it up, provided there are no creatures or objects inside it.
This strongly deviates from the earlier editions extradimensional space concept. Would it be reasonable to use this "instead of door" if placed against a wall? Or does the hole have a "flat bottom?" Can people on either side of the hole pick it up? What happens if it's placed over an aquifer (or muddy ground?)
Best Answer
Nice catch. This is really a completely different item than earlier editions. Some first thoughts:
The Really Secure wizard's lab/panic room/fortress control room. All walls are 5' thick stone; authorized personnel have access to the portable hole. Simple. When under attack, remove the hole... and there's no sign the room even exists. (Plus, it's pretty easy to hide a portable hole in a hurry.)
For the acrobatic... instant thief hidey-hole. Place on ceiling, climb up, brace against opposite sides. Makes for some dangerous infiltrators: as we all know, nobody ever looks up.
And talking of thieves... if you can close it from either side, who even needs lock-picks? Enter any secure room, from any direction, without even approaching guarded doorways. (Most medieval walls are much less than 5' thick.)
Combine with any form of visual illusion that can create the image of a wall for a well-concealed portable ambush space. If that doesn't get you surprise, nothing will.
Instead improvised pit trap. Dash around the corner with a move action, then drop it behind you. If the DM is at all nice, charging enemies may fall in.
What happens if you put it on the side of a 5' thick dam? Or better, on a storage bin or aqueduct, from underneath? (Run fast afterwards!)