A proper temporal safehouse:
- is secure, and mundane entry is controlled. You don't want Ignorant Levelers bumbling in by accident. Windows are few, frosted, screened by trees, etc.
- has a foyer, parlor, or entry space, where time travelers arrive. Only time travelers are allowed in there, to prevent breaches of the Masquerade. Ideally, you never let an enemy or the ignorant masses into the safehouse at all. Depending on your resources, it may be more realistic to assume the plumber or electrician will have to show up eventually, and prepare accordingly.
- is hidden, but not necessarily physically. There's probably a holding company, or some sort of bureaucratic obfuscation to disguise ownership. Otherwise, it'd be a simple manner for an enemy to Future Google your likely hiding places. The building needs to blend in enough that it doesn't draw too much attention, either.
- is a sturdy structure that stands for a very long time. Should your time machine break down, you can hoof it over to the Safehouse in numerous eras.
- does the majority of its business in the most remote (or most secure) era feasible. If time travel is really simple and energy-efficient, you may be able to have a safehouse in a deep cave way back in prehistory. However, if time machines consume a rare fuel, or have some other limitation to how far into the future or past they can readily transport you, then you need to put them in the eras corresponding to your interests and activities.
- keeps good records. Otherwise, rooms could get over-booked, causality violated, unfriendly guests housed simultaneously, etc. If every time traveler on your roster (along with several of their future selves) chose the same day and time to materialize, not only would it be crowded and inconvenient, it wouldn't be very safe. The records should be encoded so the enemy can't figure them out, but the records must exist.
- has one or more Temporal Panic Rooms, just in case.
- has a collection of spare parts and tools for repairing time machines. In general, it should have the best technology available, or at least the best tech you dare take to the era in question.
- has an extensive history library and wardrobe, so you can prepare for eras you plan to visit.
- is not the place you take a captured enemy! A security breach two centuries into the future means the enemy can put an a-bomb beneath your foundation before you build the place.
If the physics of your setting allows for fourth-dimensional space, pocket universes, or space beyond time, the safehouse may be hidden in such a place. In that case, some of the above traits may be skipped over, and replaced with phlebotinum or details particular to your setting.
Only a small portion of the above had any true sources, the rest is untried speculation on my part. So, please, if you see a logic hole, feel free to add to the page and address it. Likewise, if you think of a cool idea to improve on this concept, go ahead and add that as well.