The Purasjoki defensive position has been fortified in three different phases:
Phase I: 16.10. – 11.11.1939 (before the War)
Phase II: January – February 1940
Phase III: April 1940 – June 1941
During the first phase, the fortification was carried out by work units made up of reservists and conscripts from the Suomussalmi Training Centre. On both sides of the road, reinforced company defences were planned and implemented for about 300 men.
A fortified armoured barrier was built on the western bank of the river, of which remains can still be seen in the river mud. Behind the armoured barrier, a tree-lined trench was dug, which also contained tree-lined machine-gun emplacements and a few covered observation posts. Connecting trenches were dug backwards from the trench. Instead of building coffins, shrapnel-proof shelters were built on the sides of the compound trenches. When the Red Army took the defensive position, they used these shelters as accommodation, equipped with temporary stoves, as they lacked tents.
In Phase II, when the Finns had retaken the position, the combat troops repaired and replenished the fortifications as the situation allowed. Troops from JR 64 and ErP 15 took part in the work.
In phase III, the so-called “interlude”, the position was again fortified as part of the Salpa line. The work involved both soldiers and separate teams of wingmen.
It is difficult to distinguish between the structures of the three phases mentioned above, as the structures were improved and built on top of each other. Some of the original structures have been destroyed by the straightening of the road and the construction of the new bridge.