Our first 5e group had a portable stronghold in the form of the DM's own pocket dimension - a demi-plane that was limited in size, big enough to fool most who entered it into believing it was an entirely new world...
...until they circled back to the starting point whilst traveling in a straight line for several hours.
The refreshing thing about this particular demi-plane was that every time the portal was opened in a new place, it sucked in a new encounter area. The first time we witnessed this, a halfling village had mysteriously appeared in our pocket dimension - and the neighboring bugbear raider lair that habitually raided the halflings for food and slaves.
The last time we witnessed this, we were searching for the reason why the air in the dimension had been fouled. We tracked down the noxious fumes to a new 1 mile x 1 mile square area of ruined woods and the lair of a green dragon.
We couldn't fight the dragon, but my character Hellgirl came up with a plan that involved talking the dragon into voluntarily leaving our pocket dimension for 'greener' pastures. She agreed, and we dropped the green dragon, her treasure hoard (well... most of it), her clutch of eggs, and the ruined wood smack dab in the middle of an enemy city/castle.
Since this action derailed the DM's long range campaign plans for the enemy city & castle, when he left for college - after he wrapped up his campaign arc, he also announced that he would not give me permission (upon my assumption of DMing duties) to use the pocket dimension, because the NPC who claimed it as his own said we were "evil".
That's Chaotic Neutral, to you
I don't mind that he didn't want me to use his pocket dimension stronghold, since it was a little too gonzo for what I had planned for our continued exploits in the 5th edition implied setting.