When Slyvester's Dad first says they are stuck in the concrete the concrete level is just above their knees. The second time we see them stuck in the concrete the level is almost to hips, and the third and final time we see them stuck in the concrete the level is just above their waists.
Sylvester show a crest on the floor of the bunker and states it is the crest of Hamburg. In the crest there is an eagle shown. But the crest of hamburg has no eagle in it but a white castle on a red shield.
Once Slyvester and his Dad get themselves free of the concrete anybody would be able to get to the solar plates.