6 ipcs for a Blockhouse that fires 1 out of 6, and 6 more ipcs to upgrade to one that fires 2 out of 6. so that is 12 ipcs for the full version.
1 hitpoint and can not be captured (just dies if the controller loses that territory) [could also do zero hitpoints, just like AA gun, and still would not die unless territory is lost]
fires a 1 or 2 out of 6 (depending on normal or upgraded) before combat begins on all attacking land units, up to a maximum of 10 land units more than the total number of defending units (just like shore bombard, the casualties may fire their casualty shots)
once in place, it is pointing at a certain territory border and only defends against attacks coming from that border (the border can be land or sea, and therefore does not matter if it is amphibious or regular assault)
is built in place, but can only be built in territories that contain an IC or are adjacent to a territory that contains an IC. Only one blockhouse per border.