Hmm...
I don't see this happening earlier than level 9 for a divine character, or level 13 for an arcanist (although I'm sure some shenanigans can be pulled in the latter case to get it down to 9)...
If you are willing to do it strictly through items, then it would take 1 scroll each of Mage's Magnificent Mansion and Permanency (and a little handwaving, since the former normally can't be made permanent). The cost would be something like 5*15*25 + 7*13*25 + 3500*5... 21,650 gp.
An alternative is to try applying Spell Thematics to a Rope Trick spell, and then making that permanent (again, requiring handwaving). That would be free for the Rope Trick, but require a 6,500 gp scroll of permanency. This might be a bit more appropriate since you're not really adding functionality to your Rope Trick.
Higher-level alternatives would include a scroll of Genesis and a custom magic item (probably a Wonderous Item, although a Rune Circle is appropriate, too) that used Plane Shift on anyone that entered the doorway. Or maybe just the item if you put your stronghold on another plane.