This monstrous vehicle was specifically designed to combat the German heavy tanks and fortifications which the British expected to meet in the Normandy. Up to 228 mm of angled front armor made it practically immune to all German anti-tank guns. Of course this came at the cost of a very low speed of only 20 km/h. Despite all efforts it wasn't finished until the end of the war in Europe.
In FHSW the Tortoise can be compared to the T95 GMC with very high firepower and almost unmatched frontal armor. Differences are the twin machine gun turret which allows for better close defense against infantry and the ability to create smoke screens. Other tanks have to protect its flanks or you might find yourself outmaneuvered by faster tanks. Like all super-heavy tanks because of its size and slow speed it's an easy target for the enemy airforce.
In February 1944 it was proposed to create a flamethrower variant based on the Tortoise to combat heavy entrenchments and fortifications in close combat. Like all flamethrowers in FHSW the AT-17 is an excellent weapon to suppress or eliminate enemy infantry in entrenched positions. Although it's possible to also damage armored vehicles you're at a disadvantage because of the very limited range.
A28 "Infantry Cromwell"
The Cruiser Tank A28 was one of a number of proposals for improved versions of the A27M Cromwell. The A28 was a Rolls Royce design and would have featured increased armour and skirting plates over the suspension. It would have weighed 28 tons. Work on the A28 was abandoned at the paper stage, but Rolls Royce then went on to produce a series of similar but increasingly heavy designs including the A31, A32 and A35.
The Comet was designed to correct some flaws of the Cromwell with improved suspension, upped armor and more powerful armament. Since the 17 pounder wouldn't fit into the turret the gun barrel was shortened and the ammunition made more compact. It was a well-rounded tank which combined good armament and armor with a very high speed. It saw limited action during the end of the war.
In FHSW the Comet is a new weapon for the Germans to fear and unlike the Sherman Firefly it can also take a few hits. Penetration is a bit lower compared to the regular 17 pounder gun but the reload speed is higher because of the smaller round.