Archery is a powerful force in Arcane Legions. The game is won, however, not just by killing your opponent, but by holding control terrain. A unit that can survive several turns on control terrain can therefore force your opponent to engage you on more intimate terms. Units that can do this are called turtles. The Latin word for turtle is testudo, and this is the origin of the 7th Legion Testudo unit.
The key special ability for turtle units is Wall of Shields, which increases defense against ranged attacks. The four basic Roman infantry figures (Front Line Legionary, Formation Legionary, Legionary Shieldbearer, Legionary Defender) and two of the Slavic constructs (Shield Golem, Slavic Shieldwall) each have this ability to one degree or another.
In the design of a custom turtle unit, it's important to concentrate the defensive icons on figure slots by themselves. This insures that as the units take damage, they maintain their defensive capabilities as long as possible. For example, the 7th Legion Testudo below can only take four hits before losing its potency.
On the other hand, the custom 7th Legion Despero, which has an identical cost, can still present 10 defense dice against ranged attacks after taking six hits.
The 13th Legion Despero does the same thing for the Egyptians at a slightly higher cost.
Mercenaries have to pay 950 points for a low-end turtle. The resulting unit has a slightly higher defense, moves 4 steps instead of 3, is able to absorb some damage by virtue of having multi-peg figures, and can be healed using Artificer, so the increased cost is not without its benefits. The Golem Wall uses 6 Shield Golems.
The Han don't have access to Wall of Shields. Instead, they generally deploy extra artillery units. The closest a Han player can get to a defensive unit is to use a Foo Lion to absorb damage and Elusive to increase the cost of attacking. This is the philosophy behind the Ashi Defenders.
You can extend a turtle unit's life by giving it the power to heal. The strongest healing ability in the game is Lord of the Dead. An Egyptian player can build Amenhotep Testudo entirely from figures in the Starter Box. It has a defense of 12 against ranged attacks and 8 against close combat attacks. Thanks to the presence of Amenhotep III, it can bring back an average of 3 or 4 downed figures a turn.
The Artificer ability can be used to heal construct units, so another type of self-healing turtle can be created using Shield Golems and Mechanist Savros. This is the Wall of Steel.
Finally, the Healing ability can be used to bring back one figure per turn. The 7th Legion Defender uses this technique. While less sturdy than the other two units, it has the advantage of not having a commander on board, so its inevitable death is less beneficial to your opponent.
The self-healing turtle units can last quite a while, but they have no offensive power. It is also possible to build defensive units that have teeth to one extent or another. The simplest involves adding Fireburst capability to a standard 7th Legion Despero so that it can take revenge on artillery units that attack it. This is the Igneus Defender.
If you have an all-Egyptian army, you get four bonus attack dice from the Hellenic Pride ability that comes with Darius of Pontus. This is the basis for one of the most powerful infantry units in the game, the Shieldbearers of Darius. It has 8 defense against close combat, 12 defense against ranged combat, and 11 attack dice.
The heavy hitter for the Romans is Titus Petronius, who does automatic damage using Armored Rush. This is the basis for the Titus Testudo.
It is not necessary to restrict yourself to minor variations on the above themes. For example, the Elusive Testudo has less defense than a normal turtle unit, but its Elusive ability means it cost more to attack it-- 2 order points from a sortie and 3 from a formation. It uses Sniper to get revenge on units that attack from a distance.
Multi-peg figures can help to absorb damage, though they are expensive for just that reason. The Guardian Wall uses a Slavic Shieldwall to shrug off the occasional Fireburst attack. If you place an Artificer-capable unit nearby, you can bring back downed Shield Golems.
Turtle units alone will not win you a game. They are vulnerable to Sniper and Fireburst, and they generally take a long time to reach the control point due to their reduced speed. It is also important to have blocking terrain placed near the control points (as in The Valley of Honor) so that your opponent can't simply shoot everything to pieces. Nonetheless, a turtle unit is a useful tool for racking up victory points, and a good army should always have at least one.