From the website:
When the situation calls for a prolonged deployment, Space Marines can rapidly construct modular fortified structures, known as Castellum Strongholds, to use as forward operating bases on a planet’s surface.
Designed by Blake Spence, the Realm of Battle Space Marine Castellum Stronghold is a 24" x 24" (609 mm x 609 mm) tile which can fit the Citadel Realm of Battle board. The model features areas that can accommodate Tarantula Sentry turrets, and its design allows several boards to be put together, expanding the size of the base. The Castellum Stronghold will be available to pre-order now and will be despatched from Saturday 30th November.
Castellum Strongholds were used in our Isstvan V display boards at Games Day UK which you can see here.