Hi Macca, and welcome to The Tortoise Table.
That is great that you're thinking ahead about an outdoor enclosure for your Horsfield. First of all, here's a link to a little article on The Tortoise Table website that points out some of the things to consider when building an outdoor enclosure.
https://www.thetortoisetable.org.uk/tor ... /outdoors/ .
3m x 1m should be a good size, and you can make it more interesting by putting in interesting 'furniture'. For example, in my enclosure I used one of those long, curved roofing tiles to make a tunnel/hill for my tortoisees, and they love going through the tunnel and climbing over the curved top (have attached a photo). And you can use flower pots on their sides for little shelters, but cold frames are really useful, especially for warming up in the mornings. I also have one of those rabbit/guinea pig hutches with a door that also functions as a ramp -- like this
https://www.therange.co.uk/pets/small-p ... k/#3197309 It's useful because you can close up that ramp in the evening and know they are safe from predators inside and then open it in the morning, although I actually prefer now to just bring mine in at night (just a personal thing).
I don't think that squirrels and birds (depending on what species of bird) are too much of a threat, but rats and foxes are problems, so a strong mesh lid over the top (make sure it is attached to a frame or something that you can easily lift up , so that you can get inside for cleaning, removing the tortoises, etc. A good mesh lid will also prevent the tortoise climbing out (they are extraordinary climbers, and horsfields are the best of climbers (a couple of photos of Horsfields climbing are also attached). If you don't put a mesh lid on top then just make sure that there is nothing next to the wall (plant or shelter) that the tortoise could use as a ladder to climb out, and that there is an inward facing little lip on the top of the walls -- especially at the corners -- to prevent him climbing over the wall (just a small inward facing lip is enough to do it).
Regarding digging down -- you are absolutely right to consider this, as Horsfields are a burrowing species and will happily dig down, under a wall, and out. Many people dig out the enclosure to a depth of at least a foot and then lay a mesh or something similar down there. You can also sink the walls of the enclosure into the ground do a depth of a foot or so. In my enclosure the walls are only sunk in about 6 to 8 inches, but that is because my soil is heavy clay and it is very difficult for my tortoises to dig down in that -- other soils will require greater protection.
It is very useful if the enclosure is in a site that gets the morning sun -- or even better that it is south facing and gets sun for most of the day. It's also important that the ground isn't grass, but is bare earth with plants, a bit of grass, as grass gets too wet, and Horsfields in particular don't deal with cold and wet well (warm and wet is OK, but cold and wet can lead to respiratory problems), maybe a flat slab of limestone -- lots of things to provide different textures and surfaces to walk on. A water dish is also necessary, and I find that a good sized plant saucer, sunk a bit into the ground works well.
It's exciting that you're going to build your tortoise an outdoor enclosure, and I know he will appreciate it (they love the outdoors).
Nina
[attachment=0]Tortoise climbing wall.jpg[/attachment][attachment=1]Tortoise climbing fence.jpg[/attachment][attachment=2]IMG_1327 (Copy).JPG[/attachment]