This is what the BECMI edition Expert Rules from 1983 say about fighters building castles:
"A fighter with enough money may build a castle regardless of level. When a fighter reaches Name level, the character is eligible for selection as a Baron (or Baroness)."
But neither my example nor yours is quoting rules published by WotC. The historical state of the game 40 years ago doesn't reflect any changes made since WotC took the helm.
The last dedicated stronghold-related WotC product that I'm aware of is the Stronghold Builders' Guidebook from 3.x Edition. On page 4, that sourcebook says:
"Characters with enough gold and magic at their disposal can construct massive fortresses limited only by their imagination." No minimum level requirements are mentioned anywhere in the guidebook.
So, per WotC's most recent, stronghold-centric product, there are no minimum level requirements for building a stronghold. The 2024 DMG isn't required to follow this precedent, of course. But why build upon game mechanics from the 1980's when you can at least use something published in this century as a basis for your new rules system?
Here is what the 2014 DMG says:
"It’s not unusual for adventurers — especially after 10th level — to gain possession of a castle, a tavern, or another piece of property. They might buy it with their hard-won loot, take it by force, obtain it in a lucky draw from a
deck of many things, or acquire it by other means.
The Maintenance Costs table shows the per-day upkeep cost for any such property. (The cost of a normal residence isn’t included here because it falls under lifestyle expenses, as discussed in the
Player’s Handbook.) Maintenance expenses need to be paid every 30 days. Given that adventurers spend much of their time adventuring, staff includes a steward who can make payments in the party’s absence.
Total Cost per Day. The cost includes everything it takes to maintain the property and keep things running smoothly, including the salaries of hirelings. If the property earns money that can offset maintenance costs (by charging fees, collecting tithes or donations, or selling goods), that is taken into account in the table.
Skilled and Untrained Hirelings. The
Player’s Handbook explains the difference between a skilled hireling and an untrained one.
Businesses
An adventurer-owned business can earn enough money to cover its own maintenance costs. However, the owner needs to periodically ensure that everything is running smoothly by tending to the business between adventures. See the information on running a business in “
Downtime Activities” in this chapter.
Garrisons
Castles and keeps employ soldiers (use the
veteran and
guard statistics in the
Monster Manual) to defend them. Roadside inns, outposts and forts, palaces, and temples rely on less-experienced defenders (use the
guard statistics in the
Monster Manual). These armed warriors make up the bulk of a property’s skilled hirelings."