Accessible as soon as you get the ability of throwing knives (Sequence 3/Memory 4)
You can get the pouch of extra knives at any tailor shop.
The extra-large pouch is free and should give you the ability to carry 25 throwing knives instead of 20.

Will become available after completing Sequence 1/Memory 10 (once you've got Giovanni’s Assassin’s skin in his office)

1. Get the skin at any tailor shop
2. Go into the Animus Desktop (pause menu)
3. Go into Inventory/Outfits
4. Select the Altair Skin (last selection)

Will become available after completing Sequence 4.
Once Sequence 5 is loaded, you can go directly to Monterigionni
The family crypt is located at the Assassin icon position on your map

 

The Palazzo Medici map will become available after completing Sequence 4/Memory 4 (Santa Maria Novella Catacombs) and when you are in Firenze/Florence.

To replay this map after completing it (or to check to see if you have already completed it), press Start while in the game and choose "DNA". On the DNA strand, scroll to the right until you see "Secret Locations" and Choose "Templar Lair - Home Invasion".

 

The Santa Maria dei Frari map will become available after completing Sequence 7 and when you are in Venezia/Venice.

To replay this map after completing it (or to check to see if you have already completed it), press Start while in the game and choose DNA. On the DNA strand, scroll to the right until you see Secret Locations and Choose Templar Lair - Over Beams, Under Stone. 

 

The Arsenal Shipyard map will become available after completing Sequence 10 and when you are in Venezia/Venice.

In order to replay this map after completing it (or to check to see if you have already completed it), press Start while in the game and choose DNA. On the DNA strand, scroll to the right until you see Secret Locations and Choose Templar Lair - Shipwrecked. 

Will become available after completing the main story. 

Will become available after completing Sequence 12.