Although Liveman was the last to have a FLYING fortress, other series had mecha's that had that kind of role.
Turbo Builer=Turboranger, also houses their secondary mecha at the same time while Turbo Ranger (coincidence) from Maskman could only house either the five components to Great Five or Land Galaxy alone.
Magma Base=Fiveman had one very similar to Turboranger. Houses both the three components to Five Robo and Star Carrier.
Vic Trailer=Carranger, mainly houses the VRV machines. At one point, it might have housed the RV machines. But either way, it mainly houses the VRV machines. The front trailer houses V Fire. The middle trailer's right houses V Rescue and the left houses V Dump. Finally the back trailer's right houses V Police and the left houses V Dozer.
Ginga Bitus=Gingaman, houses the two five components to Ginga Phoenix, and Ginga Rhino. The top part houses the five pieces to Ginga Phoenix, while the bottom part houses the five pieces to Ginga Rhino. Ginga Phoenix and Ginga Rhino despite being robos each are made up of one Ginga beast, and are helper mecha's despite being made up of five pieces each.
Go Liners=GoGoV, one of the most creative concepts in my opinion. Each houses their respective counterparts of the pieces to either 99 Machines or Mars Machines, but they can only house one at a time. Max Shuttle could connect onto the front of it to make it fly. They are secondary sets of mecha taking the role as a fortress type of mecha. They can combine to become Grand Liner.
Providus=Timeranger, okay so he doesn't count as a fortress mecha. But he does have that kind of role. He may not house them, but he does send them off to battle. Both the Time Jets and Time Shadow. At one point sent V-Rex to the future. Also sends other stuff like that yellow ship from the first episode.
Bakuryuu Brachiosaurs=Abaranger, owned by Abare Black. Houses the three bakuryuus of the three main members as well as the four auxiliary bakuryuus.
Deka Base=Dekaranger, owned by Deka Master. Houses either the Deka Machines or the or the Pat Wings.
Gokai Machines=Gokaiger, this is kind of weird. They house each other. But anyways, Gokai Galleon houses Gokai Trailer. Gokai Trailer houses Gokai Marine. Gokai Marine houses Gokai Racer. Gokai Racer houses Gokai Jet.
Goranger, JAKQ, and Jetman were the only series without a mecha that houses their other mecha or has a similar role of some sorts.