I like the Fortress, rather want one myself.
Yes, it's 220pts base, but that gets you 4 places to hide models (the central 'walkway' seems to have 6 Firepoints on each side, so you can hide inside, not just on top.) (You can probably model more in if you're that way inclined

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You also can also easily emplace 4-5 more units on top (around gun, around silo, 1-2 on the walkway, then one on top of the tower). So that's a potential 9 units on/in the thing, with the 2 weapons to start.
The upgrades, yeah the Krakstorm is effectively an auto-take, meaning it's 250pts base.
The Heavy Bolters aren't a bad weapon - 36", Str5 is a lot better in 6th, as it can reliably glance vehicles to death - and it's nice and useful for Heavy Weapon teams.
Though it's effectiveness and worth varies on unit.
For example - the Sisters of Battle unit with the Rending Heavy bolter power can get a potential 4 extra Heavy Bolters out of it.
Long Fangs and other Heavy Weapon squads can give one to the Character, meaning he gets a Heavy Weapon with Precision Shots to throw around, rather than sitting there doing nothing.
Furthermore, the top of the tower gives a fantastic firing arc and
LOS for your Heavy Weapons.