The Kingdom of Denmark was ethnically partially German since it included the German territories of Schleswig and Holstein.
Those were ethnic Danish terrotories back then according to Christiansen. At least the countryside was fully Danish. Ethnic Danish territory extended to the south as far as to the Bay of Kiel and the mouth of river Eider.
The city of Slesvig (Germ. Schleswig) itself was ethnically mixed - there were Danes, Frisians, Saxons, Russians*, Polabians and Norwegians. Also some Poles, Germans, Slavs from Rugen island, even Greeks (Byzantines).
This is what Eric Christiansen writes about ethnic / language groups in Slesvig (page 77).
To the south and south-east of the Bay of Kiel was ethnic Slavic territory. Between the Bay of Kiel and the river Trawa (Germ. Trave) lived Wagrowie. From the Trawa river to the upper Warnawa (Germ. Warnow) river lived Warnowie. Areas from the Warnow river to the Rana (Germ. Rugen) island, Piana (Germ. Peene) river and the mouth of the Odra (Germ. Oder) river were inhabited by Veleti. On the Rana island itself lived Ranowie. To the south of those tribes lived the Serbo-Łużyczanie (Sorbs), who spoke slightly different dialects than their neighbours from the north (Sorbs spoke Eastern-Lechitic dialects while those in the north spoke Western-Lechitic dialects).
Eric Christiansen when describing those Slavic duchies & city-states along the Baltic coast up to the Polish-controlled region of Gdańsk (Pomerelia) in early 12th century, listed several large Slavic urban centers / civic centers and wrote that there was not a single city of similar size in Denmark at that time, maybe except of Slesvig.
Those main cities of the Baltic Slavs living west of the Odra river, were: Stargard (Germ. Oldenburg, Dan. Brandehuse); Ljubice (Germ. Lubeck); Racibórz (Germ. Ratzeburg); Mechlin (Germ. Mecklenburg) - Mechlin was the main city of Obodrites; Roztok (Germ. Rostock). On the Rana island were located two important cities - Arkona (Germ. Arkona) and Gardziec (Germ. Garz). At the Piana river were located Dymin (Germ. Demmin), at the Odra river Szczecin (Germ. Stettin, Dan. Burstaborg), in the area around the Szczecin Lagoon - Wołogoszcz (Germ. Wolgast), Uznam (Germ. Usedom), Lubin (Germ. Lebbin), Wolin (Germ. Wollin) and Kamień (Germ. Cammin).
There was also a big Slavic city called Vineta, the exact location of which is nowadays unknown.
As you can see all the modern German names of these cities are derived from old Slavic names (some phonetically and some semantically - like for example in case of Slav. Stargard / Germ. Oldenburg).
Indeed those were very large settlements for that time period and part of Europe. Their wealth resulted from intense trade and raids of Slavic pirates against neighbouring Christian countries. For example Slavic Szczecin had 5000 or more inhabitants, some of whom were slaves (among those slaves were many from Denmark, which was being heavily devastated by raids of Slavic pirates in period prior to the Polabian Crusades of 1147 - 1185).
Danish 12th century historian Saxo Grammaticus referred to Szczecin as an "impregnable fortress" during times when it was held by independent Slavic Pomeranians (until mid-12th century). He wrote that there was a proverb "to be safe like behind the walls of Szczecin". According to German missionary Otto from Bamberg Szczecin was inhabited by ca. 900 Slavic families / clans (plus, of course, their slaves).
One of Slavic merchants & members of the ruling class from Szczecin - certain Domisław - had his "private army" consisting of 500 armed servants and a also private fleet of 6 ships.
Indeed, Slavic Szczecin was never captured during the crusades of 1147 - 1185. But Duke of Pomerania and Szczecin Boguslav I (died 1187) was defeated in the battle of Kamien in 1185 and after that battle he agreed to become a vassal of the Danish king Canute VI (reigned 1182 - 1202). Until 1180 West Pomerania & Szczecin were vassal states of Poland (since the Polish invasion in late 1110s and early 1120s, led by duke Bolesław Krzywousty).
In German and Scandinavian sources all those Western Slavic nations were called Wends.
Regarding the border between German and Slavic territories.
At that time the northern and north-eastern border of Saxon territory was so called "limes Saxoniae" - the one hundred kilometers wide belt of almost completely uninhabited, heavily forested land, extending from the south-western banks of the Bay of Kiel to the town of Lauenburg at the Elbe river - near Hamburg (Christiansen, page 48).
To the south and west of this area lived the Saxons, while to the north and east of it lived the Slavic Wends.
As an example, the language used in the Monastic State of the Teutonic Order was a Low German dialect, not the Middle German dialect that is the Standard German language of today.
In the State of the Teutonic Order was in use not just one language, but many languages.
And even at its high in 15th century Germans were only 40% of the total population of this state. Ethnic Prussian population was gradually undergoing Germanization, but the process lasted well into 17th century.
In the eastern part of the state of the Teutonic Order majority spoke Polish or Pomeranian dialects. In the eastern part of this state, close to the border with Lithuania and near Baltic coasts, majority spoke Baltic languages.
In the southern part - modern Warmia & Mazury - Polish colonists from Mazovia were very numerous.
Regarding how many colonists came to the Teutonic State from various parts of Germany (or other Germanic-speaking countries) - I have a book about the Teutonic State & Society which contains some of such statistics.
Some paintings showing architecture & art of the Early Medieval Pagan Slavic city-states along the Baltic coast:
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There are words which carry the presage of defeat. Defence is such a word. What is the result of an even victorious defence? The next attempt of imposing it to that weaker, defender. The attacker, despite temporary setback, feels the master of situation.