Battle of Budapest
Operation Konrad

The 503rd Army Heavy Panzer Battalion (German language: Schwere Heerespanzerabteilung 503) was a German heavy Panzer Abteilung (a battalion-sized unit) equipped with Tiger I tanks and Panzer IIIs. In 1944, it was re-equipped with the new Tiger II. The 503rd saw action on the Eastern and Western Fronts during the Second World War. As with other German Heavy Panzer Detachments, it was normally not assigned to a single corps, but shuffled around according to war circumstances. Later the Panzerabteilung became part of the newly formed Panzerkorps "Feldherrnhalle" as the Feldherrnhalle Heavy Tank Battalion (Schwere Panzerabteilung Feldherrnhalle).
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In September 1944, the refitted 503rd was sent to Hungary to bolster the forces of Armeegruppe Fretter-Pico, defending the approaches to Budapest. Arriving in early October, the 3./sPzAbt 503 under Leutnant Freiherr von Rosen, provided support for Otto Skorzeny's Operation Panzerfaust, the coup which replaced the Hungarian leader, Admiral Miklós Horthy, and resulted in the Hungarians remaining as Germany's allies until the end of the War. Six King Tigers of the 3rd Company helped take the Buda Castle.
Immediately following the success of Panzerfaust, the company was returned to the detachment. The 503rd now played a major role in the Battle of Debrecen. The detachment formed the vanguard of the 23rd Panzer Division's drive to sever Cavalry Mechanized Group Pliyev's lines of communication, thus allowing the encirclement and eventual destruction of the Soviet force. During this period of action, the 503rd claimed 1,500 vehicles and 120 anti-tank guns destroyed.
In early-November, a powerful Soviet offensive pushed the Axis forces back towards Budapest. The 503rd was regularly acting in co-operation with the 60th Panzergrenadier Division Feldherrnhalle, providing a mobile reserve for the Armeegruppe, now commanded by General der Panzertruppen Hermann Balck and so renamed Armeegruppe Balck. By mid-December, the Germans had been pushed back to Budapest, and the Soviets were probing the outskirts of the city. The 503rd, along with the Feldherrnhalle, having lost many of its Tigers to combat and to mechanical issues, was engaged in fighting off Soviet attacks on Budapest's government district.
On December 21, the 503rd, having lost virtually all of its armour, was renamed to schwere-Panzer-Abteilung Feldherrnhalle (abbreviated sPzAbt FHH) and officially attached to the 60th Panzergrenadier Division Feldherrnhalle.
On 31 December, the Soviets completed the encirclement of Budapest and the siege of the city began. During the Battle of Budapest, the majority of the FHH along with roughly 70,000 German and Hungarian troops of the IX SS Mountain Corps were trapped. The remnants of the detachment saw heavy fighting during the final Soviet assaults to annihilate the trapped forces. The 503rd took part in several failed attempts to relieve Budapest, codenamed Operation Konrad.

Orders of battle
October 1944 - Debrecen
Abteilung Stab Zug (3x Tiger II ausf B, 6x SdKfz 250)
1./sPzAbt 503 (14x Tiger II ausf B)
2./sPzAbt 503 (14x Tiger II ausf B)
3./sPzAbt 503 (14x Tiger II ausf B)

For more information on the history of the 503rd Army Heavy Panzer Battalion please check military-history.fan..avy_Panzer_Battalion