Russia's Kaliningrad region has announced the castle is up for sale.
This is the first time Belarus has joined Russia and Serbia for their annual military drills, which will feature some of Moscow's new military equipment.
Russia's manoeuvres in Ukraine have sparked concern in one of the few European countries not to join a U.S.-led military alliance.
The country has stopped short of joining NATO.
Russia's upcoming Zapad drill was described as a "simulated attack" by Lithuania's government.
Poland has already agreed to purchase the system, while Sweden will be next to experience it firsthand in a joint drill with U.S.
Russian jets have been caught near NATO skies twice recently.
Russian helicopters blasted their targets not far from where the U.S. and NATO allies have just begun fortifying their air defenses.
Russia's upcoming nearby drill has been described as a "simulation" for an attack on its Baltic neighbors.
Poland has long called for U.S. backing in deterring Moscow, and Russian missiles are set to arrive near its border.
Moscow has sent 2,500 troops to its western border region to practice landing in unfamiliar territory.
The border between Poland and Lithuania is strategically important because its seizure would cut off the Baltic states from the rest of the Western alliance.
A crowd of Russian aircraft, including Tu-160 supersonic bombers, had three air forces tracking them.
Senators Ben Cardin and John McCain have introduced a bipartisan bill.
"Everything is about Russia taking back territory that was historically Soviet."
Jens Stoltenberg spoke as 4,000 troops prepare to shore up defenses around the Baltic Sea.
Putin can't afford to cut pensions, salaries and social subsidies to continue financing his hybrid wars.